Bloom began his career by tackling the formidable legacy of T.S. Eliot, who had dismissed the English Romantic poets as undisciplined nature-worshippers. Bloom construed the Romantic poets' visions of immortality as rebellions against nature, and argued that an essentially Romantic imagination was still at work in the best modernist poets.

Having restored the Romantics to critical respectability, Bloom advanced a more general theory of poetry. His now-famous The Anxiety of Influence argued that any strong poem is a creative "misreading" of the poet's predecessor. The book raised, as the poet John Hollander wrote, "profound questions about... how the prior visions of other poems are, for a true poet, as powerful as his own dreams and as formative as his domestic childhood." In addition to developing this theory, Bloom wrote several books on sacred texts. In The Book of J, he suggested that some of the oldest parts of the Bible were written by a woman.

The Book of J was a bestseller, but it was the 1994 publication of The Western Canon that made the critic-scholar a household name. In it, Bloom decried what he called the "School of Resentment" and the use of political correctness as a basis for judging works of literature. His defense of the threatened canon formed, according to The New York Times, a "passionate demonstration of why some writers have triumphantly escaped the oblivion in which time buries almost all human effort."

Bloom placed Shakespeare along with Dante at the center of the Western canon, and he made another defense of Shakespeare's centrality with Shakespeare: The Invention of the Human, an illuminating study of Shakespeare's plays. How to Read and Why (2000) revisited Shakespeare and other writers in the Bloom pantheon, and described the act of reading as both a spiritual exercise and an aesthetic pleasure.

Recently, Bloom took up another controversial stance when he attacked Harry Potter in an essay for The Wall Street Journal. His 2001 book Stories and Poems for Extremely Intelligent Children of All Ages advanced an alternative to contemporary children's lit, with a collection of classic works of literature "worthy of rereading" by people of all ages.

The poet and editor David Lehman said that "while there are some critics who are known for a certain subtlety and a certain judiciousness, there are other critics... who radiate ferocious passion." Harold Bloom is a ferociously passionate reader for whom literary criticism is, as he puts it, "the art of making what is implicit in the text as finely explicit as possible."