American writer, biographer, and book editor, best known for his acclaimed literary biographies of Mark Twain (Twain, Mark), Lincoln Steffens (Steffens, Lincoln), and Walt Whitman (Whitman, Walt).

Kaplan grew up in New York City and graduated from Harvard (B.S., 1944). He left graduate school in 1946 and worked for a publishing house, eventually becoming a senior editor. In that capacity he worked with such authors as Bertrand Russell (Russell, Bertrand), Will Durant (Durant, Will; and Durant, Ariel), Níkos Kazantzákis (Kazantzákis, Níkos), and the sociologist C. Wright Mills (Mills, C. Wright).

In 1959 Kaplan left publishing to write his first book, a biography of Mark Twain (Twain, Mark) entitled Mr. Clemens and Mark Twain (1966), which won both a Pulitzer Prize and a National Book Award (National Book Awards). Kaplan began the biography with Clemens at age 31 rather than at the beginning of his life, a device that was later emulated by other biographers. Also well regarded are Kaplan's Lincoln Steffens: A Biography (1974), about the prominent journalist and muckraker of the late 19th and early 20th centuries, and Walt Whitman: A Life (1980).

Kaplan lectured at Harvard and at Emerson College, Boston, and was biographer in residence at the Institute for Modern Biography at Griffith University, Brisbane, Australia. He edited several anthologies and was general editor for the 16th edition of Bartlett's Familiar Quotations (1992).