1937 February 6, play-novelette, Of Mice and Men published by Covici-Friede; Summer, first trip to Europe and Russia; September, The Red Pony , three connected stories, published by Covici-Friede; November 23, New York opening of the play Of Mice and Men (207 performances).

1938 April, Their Blood Is Strong, a nonfiction account of the migrant labor problem in California, published by the Simon J. Lubin Society (San Francisco); May, receives the New York Drama Critics Circle Award for the play Of Mice and Men; September, short story collection, The Long Valley, incorporating The Red Pony (1937), published by Viking (New York), where Pascal Covici became an editor after the bankruptcy of his own firm.

1939 April, The Grapes of Wrath, his greatest critical success, published by Viking, provoking both great popular acclaim and violent political condemnation for its depiction of Oklahoma migrants and California growers, as well as for its alleged "vulgar" language and socialist bias. December, film of Of Mice and Men released.

1940 January, film of The Grapes of Wrath released; March 11 - April 20, marine expedition in the Gulf of California with Ricketts; Spring, receives the National Book Award and the Pulitzer Prize for The Grapes of Wrath; Summer, documentary film about living conditions in rural Mexico, The Forgotten Village.

1941 Spring, separated from Carol; Fall, moves to New York City with singer Gwyndolyn Conger; December 5, Sea of Cortez, written with Edward Ricketts, published by Viking.

1942 March, sued for divorce by Carol; March 6, novel, The Moon Is Down published by Viking; April 8, New York opening of the play The Moon Is Down; May, film of Tortilla Flat released; November 27, Bombs Away published by Viking.