Smith's father encountered financial difficulties upon his return to England and he was forced to sell some of the family's holdings and to marry the wealthy Henrietta Meriton in 1765. Smith entered society at the age of twelve, leaving school and being tutored at home. On 23 February 1765, at the age of fifteen, she married Benjamin Smith, the son of Richard Smith, a wealthy West Indian merchant and a director of the East India Company. The proposal was accepted for her by her father; forty years later, Smith condemned her father's action, which she wrote had turned her into a "legal prostitute".
Smith's marriage was unhappy. She detested living in commercial Cheapside (the family later moved to Southgate and Tottenham) and argued with her in-laws, who she believed were unrefined and uneducated. They, in turn, mocked her for spending time reading, writing, and drawing. Even worse, Benjamin proved to be violent, unfaithful, and profligate. Only her father-in-law, Richard, appreciated her writing abilities, although he wanted her to use them to further his business interests. Richard Smith owned plantations in Barbados and he and his second wife brought five slaves to England, who, along with their descendants, were included as part of the family property in his will. Although Charlotte Smith later argued against slavery in works such as The Old Manor House (1793) and "Beachy Head", she herself benefited from the income and slave labor of Richard Smith's plantations.
In 1766, Charlotte and Benjamin had their first child, who died the next year just days after the birth of their second, Benjamin Berney (1767–77). Between 1767 and 1785, the couple had ten more children: William Towers (1768–), Charlotte Mary (c. 1769–), Braithwaite (1770–), Nicholas Hankey (1771–1837), Married Anni Petroose (1779-1843), Charles Dyer (1773–), Anna Augusta (1774–94), Lucy Eleanor (1776–), Lionel (1778–1842), Harriet (c. 1782–), and George (c.1785–). Only six of Smith's children survived her.
Smith assisted in the family business that her husband had abandoned by helping Richard Smith with his correspondence. She persuaded Richard to set Benjamin up as a gentleman farmer in Hampshire and lived with him at Lys Farm from 1774 until 1783. Worried about Charlotte's future and that of his grandchildren and concerned that his son would continue his irresponsible ways, Richard Smith willed the majority of his property to Charlotte's children. However, because he had drawn up the will himself, the documents contained legal problems. The inheritance, originally worth nearly £36,000, was tied up in chancery after his death in 1776 for almost forty years. Smith and her children saw little of it. (It has been proposed that this real case may have inspired the famous fictional case of Jarndyce and Jarndyce, in Dickens's Bleak House.) In fact, Benjamin illegally spent at least a third of the legacy and ended up in King's Bench Prison in December 1783. Smith moved in with him and it was in this environment that she wrote and published her first work, Elegiac Sonnets (1784). Elegiac Sonnets achieved instant success, allowing Charlotte to pay for their release from prison. Smith's sonnets helped initiate a revival of the form and granted an aura of respectability to her later novels (poetry was considered the highest art form at the time). Smith revised Elegiac Poems several times over the years, eventually creating a two-volume work.