D. Keith Mano praised Grendel lavishly in The New York Times Book Review, saying: "John Gardner's 'Grendel' is myth itself: permeated with revelation, with dark instincts, with swimming, riotous universals. The special profundity of Gardner's vision or visions is so thought-fertile that it shunts even his fine poet's prose to a second importance". Another Times reviewer, Richard Locke, declared the novel "an extraordinary achievement – very funny, original and deft, altogether lovable, poignant, rich with thought and feeling". Kirkus Reviews also reviewed Grendel favorably, saying "Gardner demonstrates his agility at juggling metaphysical notions while telling a diverting tale".
The book was nominated for the 1972 Mythopoeic Award for best novel.
It was given special mention by Diana Athill in her memoir Stet, covering her decades as an editor with UK publisher Andre Deutsch: "Having to read Beowulf almost turned me against Oxford, so when a New York agent offered me this novel I could hardly bring myself to open it. If I hadn't, I would have missed a great pleasure – a really powerful feat of imagination."