Clark served as poetry editor of The Paris Review from 1963 to 1973 and published numerous volumes of poetry with Black Sparrow Press, including a verse biography: Junkets on a Sad Planet: Scenes from the Life of John Keats (1994). His literary essays and reviews have appeared in The New York Times, Times Literary Supplement, Los Angeles Times, San Francisco Chronicle, London Review of Books, and many other journals; some of his essays on contemporary poetry have been collected in The Poetry Beat: Reviewing the Eighties. From 1987 to 2008 he taught Poetics at New College of California.

Currently residing in California, Clark remains an active writer producing poetry, fiction, and nonfiction. In 1991, he published a biography of Charles Olson, one of his poetic mentors, entitled Charles Olson: The Allegory of a Poet’s Life (Norton: 1991).

Clark's biographies of literary figures of the 20th century include:

• Late Returns: A Memoir of Ted Berrigan,
• Jack Kerouac: A Biography,
• Robert Creeley and the Genius of the American Common Place,
• Charles Olson: The Allegory of a Poet’s Life, and
• Edward Dorn: A World of Difference