Lowell's early poetry was "characterized by its Christian motifs and symbolism, historical references, and intricate formalism." His first three volumes were notably influenced by the New Critics, particularly Lowell's former professors, John Crowe Ransom and Allen Tate.
Lowell's first book of poems, Land of Unlikeness (1944) was also highly influenced by Lowell's conversion to Catholicism, leading Tate to call Lowell "a Catholic poet" in his introduction to the volume. The book was published by a small press as a limited edition, but still received some "decent reviews" from major publications like Poetry and Partisan Review.
In 1946, Lowell received wide acclaim for his next book, Lord Weary's Castle, which included five poems slightly revised from Land of Unlikeness and thirty new poems. Among the better-known poems in the volume are "Mr. Edwards and the Spider" and "The Quaker Graveyard in Nantucket." Lord Weary's Castle was awarded the Pulitzer Prize in 1947. That year, Lowell also was awarded a Guggenheim fellowship.
Randall Jarrell gave Lord Weary's Castle high praise, writing, "It is unusually difficult to say which are the best poems in Lord Weary's Castle: several are realized past changing, successes that vary only in scope and intensity--others are poems that almost any living poet would be pleased to have written ... [and] one or two of these poems, I think, will be read as long as men remember English."
Following soon after his success with Lord Weary's Castle, Lowell served as the Consultant in Poetry to the Library of Congress from 1947−1948 (a position now known as the U.S. Poet Laureate).
In 1951, Lowell published The Mills of the Kavanaughs, which centered on its epic title poem and failed to receive the high praise that his previous book had received. Although it received a generally positive review in The New York Times, Randall Jarrell gave the book a mixed review. Although Jarrell liked the shorter poems, he thought the epic title poem didn't work, stating ""The people [in 'The Mills of the Kavanaughs'] too often seem to be acting in the manner of Robert Lowell, rather than plausibly as real people act . . .I doubt that many readers will think them real."  Following The Mills of the Kavanaughs, Lowell hit a creative roadblock and took a long break from publishing. However, by the end of the decade, he started writing again and changed stylistic direction with his next book of verse, Life Studies (1959), which won the National Book Award for poetry in 1960 and became the most influential book that Lowell would ever publish. In his acceptance speech for the National Book Award, Lowell famously divided American poetry into two camps: the "cooked" and the "raw." This commentary by Lowell was made in reference to the popularity of Allen Ginsberg and the Beat Generation poets and was a signal from Lowell that he was trying to incorporate some of their "raw" energy into his own poetry.
The poems in Life Studies were written in a mix of free and metered verse, with much more informal language than he had used in his first three books. It marked both a turning point in Lowell's career and a turning point for American poetry in general. Because many of the poems documented details from Lowell's family life and personal problems, one critic, M.L. Rosenthal, labeled these poems "confessional" in a review of Life Studies that first appeared in The Nation magazine. Lowell's editor and friend Frank Bidart notes in his afterword to Lowell's Collected Poems, "Lowell is widely, perhaps indelibly associated with the term 'confessional,'" though Bidart questions the accuracy of this label. But for better or worse, this label stuck and led to Lowell being grouped together with other influential confessional poets like Lowell's former students W. D. Snodgrass, Sylvia Plath, and Anne Sexton.
Lowell followed Life Studies with Imitations (1961), a volume of loose translations of poems by classical and modern European poets, including Rilke, Montale, Baudelaire, Pasternak, and Rimbaud, for which he received the 1962 Bollingen Poetry Translation Prize. However, critical response to Imitations was mixed and sometimes hostile (as was the case with Vladimir Nabokov's public response to Lowell's Mandelstam translations). In a review of Lowell's Collected Poems, the poet Michael Hofmann wrote that although he thought Life Studies was Lowell's best book, Imitations was Lowell's most "pivotal book," arguing that the book "marks the entry into his work of what one might term ‘international style’, something coolly open to not-quite-English." In the book's introduction, Lowell explained that his idiosyncratic translations should be thought of as "imitations" rather than strict translations since he took many liberties with the originals, trying to "do what [his] authors might have done if they were writing their poems now and in America."
Also in 1961, Lowell published his English translation of the French verse play Phèdre by 17th century playwright Jean Racine. Lowell changed the spelling of the title of the play to Phaedra. This translation was Lowell's first attempt at translating a play, and the piece received a generally positive review from The New York Times. Broadway director and theater critic Harold Clurman wrote that Lowell's Phaedra was "a close paraphrase of Racine with a slightly Elizabethan tinge; it nevertheless renders a great deal of the excitement--if not the beauty--which exists in the original." Clurman accepted Lowell's contention that he wrote his version in a meter reminiscent of Dryden and Pope, and while Clurman conceded that the feel of Lowell's version was very different from the feel of French verse, Clurman considered it to be like "a finely fiery English poem," particularly in passages where "Lowell's muse took flame from Racine's shade."
Lowell's next book of original verse For the Union Dead (1964) was widely praised, particularly for its title poem, which invoked Allen Tate's "Ode to the Confederate Dead." Helen Vendler states that the title poem in the collection "honors not only the person of [the Civil War hero] Robert Gould Shaw, but also the stern and beautiful memorial bronze bas-relief [depicting Shaw and the all-black 54th Massachusetts Volunteer Infantry Regiment]. . . which stands opposite the Boston State House." Paula Hayes observes that, in this volume, "Lowell turned his attention toward ecology, Civil Rights, and labor rights . . .often to the effect of combining the three concerns." For the Union Dead was Lowell's first book since Life Studies to contain all original verse (since it did not include any translations), and in writing the poems in this volume, Lowell built upon the looser, more personal style of writing that he had established in the final section of Life Studies. Lowell also wrote about a number of world historical figures in poems like "Caligula," "Jonathan Edwards in Western Massachusetts," and "Lady Raleigh's Lament," and he combined personal and public concerns in poems like the title poem and "Fall 1961" which addressed Lowell's fear of nuclear war during the height of the Cold War.
In 1964, Lowell also wrote three, one-act plays that were meant to be performed together as a trilogy, titled The Old Glory. The first two parts, "Endecott the Red Cross" and "My Kinsman, Major Molineux" were stage adaptations of short stories by Nathaniel Hawthorne, and the third part, "Benito Cereno," was a stage adaptation of a novella by Herman Melville. The Old Glory was produced off-Broadway at the American Place Theatre in New York City in 1964 and directed by Jonathan Miller. It won five Obie Awards in 1965 including an award for "Best American Play." The play was published in its first printing in 1965 (with a revised edition following in 1968).
In 1967, Lowell published his next book of poems, Near the Ocean. With this volume, Lowell returned to writing more formal, metered verse. The second half of the book also shows Lowell returning once again to writing loose translations (including verse approximations of Dante, Juvenal, and Horace). The best known poem in this volume is "Waking Early Sunday Morning," which was written in eight-line tetrameter stanzas (borrowed from Andrew Marvell's poem "Ode upon Cromwell's Return from Ireland") and showed contemporary American politics overtly entering into Lowell's work. Ian Hamilton noted that "'Waking Early Sunday Morning' is now thought of as a key 'political poem' of the 1960s."
Pity the planet, all joy gone from this sweet volcanic cone; peace to our children when they fall in small war on the heels of small war—until the end of time to police the earth, a ghost orbiting forever lost in our monotonous sublime.“ ” From "Waking Early Sunday Morning," Near the Ocean (1967)
During 1967 and 1968, Lowell experimented with a verse journal, first published as Notebook 1967-68 (and later republished in a revised and expanded edition, titled Notebook). Lowell referred to these fourteen-line poems as sonnets although they sometimes failed to incorporate regular meter and rhyme (both of which are defining features of the sonnet form); however, some of Lowell's sonnets (particularly the ones in Notebook 1967-1968) were written in blank verse with a definitive pentameter and a small handful also included rhyme. Regarding the issue of meter in these poems, Lowell wrote "My meter, fourteen line unrhymed blank verse sections, is fairly strict at first and elsewhere, but often corrupts in single lines to the freedom of prose."
In the Notebook poems, Lowell included the poem "In The Cage," a sonnet that he had originally published in Lord Weary's Castle. He also included revised, sonnet versions of the poems "Caligula" and "Night-Sweat" (originally published in For the Union Dead) and of "1958" and "To Theodore Roethke: 1908-1963" (originally published in Near the Ocean). In his "Afterthought" at the end of Notebook 1967-1968, Lowell explained the premise and timeline of the book:
This is not my diary, my confession, not a puritan's too literal pornographic honesty, glad to share private embarassment, and triumph. The time is a summer, an autumn, a winter, a spring, another summer; here the poem ends, except for turned-back bits of fall and winter 1968 ... My plot rolls with the seasons. The separate poems and sections are opportunist and inspired by impulse. Accident threw up subjects, and the plot swallowed them--famished for human chances. I lean heavily to the rational, but am devoted to surrealism.
In this same "Afterthought" section, Lowell acknowledges some of his source materials for the poems, writing, "I have taken from many books, used the throwaway conversational inspirations of my friends, and much more that I idly spoke to myself." Some of the sources and authors he cites include Jesse Glenn Gray's The Warriors, Simone Weil's Half a Century Gone, Herbert Marcuse, Aijaz Ahmad, R.P. Blackmur, Plutarch, Stonewall Jackson, and Ralph Waldo Emerson.
Steven Gould Axelrod wrote that, "[Lowell's concept behind the sonnet form] was to achieve the balance of freedom and order, discontinuity and continuity, that he [had] observed in [Wallace] Stevens's late long poems and in John Berryman's Dream Songs, then nearing completion. He hoped that his form ... would enable him 'to describe the immediate instant,' an instant in which political and personal happenings interacted with a lifetime's accumulation of memories, dreams, and knowledge." Lowell liked the new form so much that he reworked and revised many of the poems from Notebook and used them as the foundation for his next three volumes of verse, all of which employed the same loose, fourteen-line sonnet form.
In 1969, Lowell made his last foray into dramatic work with the publication of his prose translation of the ancient Greek play Prometheus Bound by Aeschylus. The play was directed by Jonathan Miller, who had previously directed Lowell's The Old Glory, at the Yale School of Drama.
In 1973, Lowell published three books of sonnets. The first two, History and For Lizzie and Harriet, consisted of revised and reordered versions of sonnets from Notebook. History included poems that primarily dealt with world history from antiquity up to the mid-20th century (although the book did not always follow a linear or logical path and contained many poems about Lowell's friends, peers, and family). The second book, For Lizzie and Harriet, included poems that described the breakdown of his second marriage and contained poems that were supposed to be in the voices of his daughter, Harriet, and his second wife, Elizabeth. Finally, the last work in Lowell's sonnet sequence, The Dolphin (1973), which won the 1974 Pulitzer Prize, included poems about his daughter, his ex-wife, and his new wife Caroline Blackwood whom he had affectionately nicknamed "Dolphin." The book only contained new poems, making it the only book in Lowell's 1973 sonnet trilogy not to include revised and reordered poems from Notebook.
A minor controversy erupted when Lowell admitted to having incorporated (and altered) private letters from his ex-wife, Elizabeth Hardwick into poems for The Dolphin. He was particularly criticized for this by his friends Adrienne Rich and Elizabeth Bishop. Bishop presented Lowell with an argument against publishing The Dolphin. In a letter to Lowell regarding The Dolphin, dated March 21, 1972, before he'd published the book, Bishop praised the writing, saying, "Please believe that I think it is wonderful poetry." But then she stated, "I'm sure my point is only too plain ... Lizzie [Hardwick] is not dead, etc.--but there is a 'mixture of fact & fiction' [in the book], and you have changed [Hardwick's] letters. That is 'infinite mischief,' I think ... One can use one's life as material--one does anyway--but these letters--aren't you violating a trust? IF you were given permission--IF you hadn't changed them ... etc. But art just isn't worth that much." Adrienne Rich responded to the controversy quite differently. Instead of sending Lowell a private letter on the matter, she publically criticized Lowell and his books The Dolphin and To Lizzie and Harriet in a review that appeared in the American Poetry Review and that effectively ended the two poets' long-standing friendship. Rich called the poems "cruel and shallow." 
Lowell's sonnets from the Notebook poems through to The Dolphin met with mixed responses upon publication, and critical consensus on the poems continues to be mixed. Some of Lowell's contemporaries, like Derek Walcott and William Meredith, praised the poems. Meredith wrote about Notebook: 1967-68, "Complex and imperfect, like most of the accomplishments of serious men and women today, Robert Lowell's Notebook 1967-68 is nevertheless a beautiful and major work." But a review of History, For Lizzie and Harriet, and The Dolphin by Calvin Bedient in The New York Times was mostly negative. Bedient wrote, "Inchoate and desultory, the poems never accumulate and break in the great way, like a waterfall seen from the lip, more felt than seen. In truth, they are under no pressure to go anywhere, except to the 14th line. Prey to random associations, they are full of false starts, fractures, distractions." The sonnets also received a negative review by William Pritchard in the Hudson Review. Since the release of Lowell's Collected Poems in 2003, a number of critics and poets have praised the sonnets, including Michael Hofmann, William Logan, and Richard Tillinghast (though Logan and Hofmann note that they both strongly preferred the original Notebook versions of the sonnets over the revised versions that Lowell published in History and To Lizzie and Harriet). Still the sonnet volumes have also received recent negative responses as well. In an otherwise glowing review of Lowell's Collected Poems, A.O. Scott wrote, "The three sonnet sequences Lowell published in 1973 ... occupy nearly 300 pages, and reading them, one damn sonnet after the other, induces more stupor than rapture." And in her review of the Collected Poems, Marjorie Perloff called the sonnet poems "trivial and catty," considering them to be Lowell's least important volumes.
Lowell published his last volume of poetry, Day by Day, in 1977, the year of his death. In May 1977, Lowell won the $10,000 National Medal for Literature awarded by the American Academy and Institute of Arts and Letters, and Day by Day was awarded that year's National Book Critics Circle Award for poetry. In a documentary on Lowell, Anthony Hecht said that "[Day by Day was] a very touching, moving, gentle book, tinged with a sense of [Lowell's] own pain and the pain [he'd] given to others." It was Lowell's only volume to contain nothing but free verse. In many of the poems, Lowell reflects on his life, his past relationships, and his own mortality. The best-known poem from this collection is the last one, titled "Epilogue," in which Lowell reflects upon the "confessional" school of poetry with which his work was associated. In this poem he wrote,
But sometimes everything I write
with the threadbare art of my eye seems a snapshot, lurid, rapid, garish, grouped, heightened from life, yet paralyzed by fact. All's misalliance.Yet why not say what happened?
In her article "Intimacy and Agency in Robert Lowell's Day by Day," Reena Sastri notes that critical response to the book has been mixed, stating that during the initial publication of the book, some critics considered the book "a failure" while other critics, like Helen Vendler and Marjorie Perloff, considered it a success. She also notes that in reviews of Lowell's Collected Poems in 2003, Day by Day received mixed responses or was ignored by reviewers. Sastri herself argues that the book is under-appreciated and misunderstood. The book has received significant critical attention from Helen Vendler who has written about the book in essays and in her book Last Looks, Last Books: Stevens, Plath, Lowell, Bishop, Merrill (2010). In her essay "Robert Lowell's Last Days and Last Poems," she defended the book from attacks following its publication in reviews like the one written by the poet Donald Hall in which Hall called the book a failure, writing that he thought the book was "as slack and meretricious as Notebook and History which preceded it." Vendler argued that most critics of the book were disappointed because Lowell's last book was so much different from any of his previous volumes, abandoning ambitious metaphors and political engagement for more personal snapshots. She wrote, "Now [Lowell] has ended [his career], in Day by Day, as a writer of disarming openness, exposing shame and uncertainty, offering almost no purchase to interpretation, and in his journal-keeping, abandoning conventional structure, whether rhetorical or logical. The poems drift from one focus to another; they avoid the histrionic; they sigh more often than they expostulate. They acknowledge exhaustion; they expect death." She praises some of Lowell's descriptions, particularly his descriptions of impotence, depression, and old age.
In 1987, Lowell's longtime editor, Robert Giroux edited Lowell's Collected Prose. The collection included Lowell's book reviews, essays, excerpts from an unfinished autobiography, and an excerpt from an unfinished book, tentatively titled A Moment in American Poetry.
Lowell's Collected Poems, edited by Frank Bidart and David Gewanter, was published in 2003. The Collected Poems was a very comprehensive volume that included all of Lowell's major works with the exception of Notebook 1967-1968 and Notebook. However, many of the poems from these volumes were republished, in revised forms, in History and For Lizzie and Harriet. Soon after the publication of The Collected Poems, The Letters of Robert Lowell, edited by Saskia Hamilton, was published in 2005. Both Lowell's Collected Poems and his Letters received positive critical responses from the mainstream press.