Eastman was born in upstate New York to parents who were protestant ministers. He graduated from Williams College in Massachusetts in 1915 and pursued a doctorate in philosophy at Columbia University but withdrew before accepting it. He studied under pragmatic philosopher John Dewey. Eastman lived in Greenwich Village with his sister and became active in the women's suffrage movement.
As editor of The Masses, he published the work of novelist Sherwood Anderson, Marxist Louise Bryant, poets Amy Lowell and Carl Sandburg, and Taos arts patron Mabel Dodge Luhan. He advocated free love and birth control, denounced America's participation in World War I, and supported socialist Jack Reed's trip to Russia to report on the Bolshevik Revolution. He created, with his sister, The Liberator magazine and published the work of Ernest Hemingway, John Dos Passos, E.E. Cummings, and Helen Keller. He lived for almost two years in the Soviet Union and watched the power plays between Joseph Stalin and Leon Trotsky. He became Trotsky's friend. But his views of socialism began to change after his visit to Soviet Russia.
During the Depression, Eastman became critical of such socialist thinkers Karl Marx and Georg Wilhelm Friedrich Hegel. He became friends with British economist Friedrich Hayek and arranged for his book The Road to Serfdom (1943) to be serialized in Reader's Digest. At first, he supported Sen. Joseph McCarthy's anti-Communist activism, but later criticized him as being too reactionary. Nevertheless, he grew to believe the Bolshevik Revolution had produced tyranny instead of freedom. Eastman was among the first contributing editors for the National Review and served on its Board of Associates but he eventually resigned because the magazine's pro-Christian leanings didn't jibe with his atheism. Eastman opposed the Vietnam War.
Among his books and articles are Enjoyment of Poetry (1913), The Sense of Humor (1921), Leon Trotsky: The Portrait of a Youth (1925), Since Lenin Died (1925), Marx and Lenin: The Science of Revolution (1927), Enjoyment of Laughter (1936), Reflections on the Failure of Socialism (1955), and Seven Kinds of Goodness (1967).