His early art talent was encouraged by his grandmother, and at Stanford University, he studied oil painting with Victor Arnautoff and Daniel Mendelowitz. He served in the active reserves during World War II and attended the University of California, Berkeley where he studied with but was not greatly influenced by abstract expressionist, Hans Hofmann. He credited Edward Hopper, Paul Cezanne, and Arshile Gorky as major influences on his painting.
In 1959, he attended the University of New Mexico, taught at the University of Illinois in 1952, and returned to Berkeley in late 1953. There he painted from the model with David Park and Elmer Bischoff, but feeling constricted he began driving around seeking outdoor landscape subjects and also began his series of peopled interiors.
His renunciation of abstraction for more realistic figures was the beginning of the Bay Area figurative school, an alternative to the mainstream. A typical Diebenkorn figure is usually a woman in a room, often with his wife, Phyllis, posed as the model. Usually the figures are expressionless, lonely, and acquiescent seeming.
However, in the late 1960s, he returned to abstraction, shifting planes of color, inspired by seeing Matisses at the Hermitage in Russia. This influence led to his "Ocean Park" series, begun in 1967 after moving to Santa Monica. In the early 1980s, he began the closing chapter to his work, which was the depiction of heraldic emblems in collage and gouache.