Justin Kaplan has written or edited numerous books, including the biographies Mr. Clemens and Mark Twain, which won both the Pulitzer Prize for Biography and the National Book Award in Arts and Letters, and Walt Whitman: A Life, which won a second National Book Award. He is General Editor of the seventeenth edition of Bartlett's Familiar Quotations. His most recent biography is Lincoln Steffens (2004).

He has been married to the author Anne Bernays since 1954. Together they wrote Back Then: Two Lives in 1950s New York, a double memoir. They live in Cambridge and Truro, Massachusetts.