Richard Dysart was born to Alice (née Hennigar) and Douglas Dysart, a podiatrist, near Boston, Massachusetts on March 30, 1929. Dysart was raised in Skowhegan, Maine and Augusta, Maine. He attended Gould Academy in Bethel, Maine. At the encouragement of his mother, Dysart performed in summer stock at the Lakewood Theater near Skowhegan. He also worked at a local radio station.

The scene in which his L.A. Law character, Leland McKenzie, who was the patriarchal and stiff founder of a successful law practice, was revealed to be in bed with competitor Rosalind Shays (Diana Muldaur) was ranked as the 38th greatest moment in television in an issue of Egg magazine. He earned one Emmy Award, and three more nominations, for his role on L.A. Law.

Dysart's acting career began on the stage. He was a founding member of the American Conservatory Theater, San Francisco, California, which began in 1960. He performed on Broadway in All in Good Time (1965), A Place Without Doors (1970-1971), The Little Foxes revival as Horace Giddens (1967–1968), and That Championship Season (1972–1974).

Dysart received a Drama Desk Award in 1972 and an Emmy Award in 1992.

His other movie credits included roles in The Hindenburg, An Enemy of the People, Prophecy, The Thing, Pale Rider, and Day One (with L.A. Law co-star Michael Tucker). Dysart created the role of Coach in the original Broadway production of Jason Miller's Pulitzer Prize winning play, That Championship Season in 1973. He appeared in an episode of the 1976 CBS television series Sara.

In 1979, Dysart portrayed a good-hearted physician in the film Being There, starring Peter Sellers and Melvyn Douglas. In 1980, he played Abraham Lincoln's Secretary of War Edwin Stanton in the television film The Ordeal of Dr. Mudd. He voiced the kindly miner Uncle Pom in the Disney English-language version of Hayao Miyazaki's 1986 adventure classic Castle in the Sky and the character of Cogliostro on Todd McFarlane's Spawn: The Animated Series, which aired on HBO.

Dysart was married three times. The first two marriages resulted in divorce. He and his third wife, Artist Kathryn Jacobi, were married from 1987 until his death. He had no children of his own, but had a stepson from his third wife and two step-grandchildren.

In 1990, Dysart was awarded an Honorary Doctorate of Law.

Dysart died at home in Santa Monica, California on April 5, 2015, after a long bout with cancer. He was 86 years old.

He earned both bachelor’s and master’s in speech communication from Emerson College in Boston, although his undergraduate education was interrupted due to his Service for four years in the United States Air Force during the Korean War. At Emerson he performed on stage, and he was a class officer and student government vice-president. He was a brother of the Phi Alpha Tau fraternity. He also studied at George Washington University. He returned for his master's degree later, completing it in 1981.