Farrell was born in St. Paul, Minnesota, one of four children. When he was two years old, his family moved to Hollywood, California, where his father worked as a movie studio carpenter. Farrell graduated from Hollywood High School, served in the United States Marine Corps, and worked at various jobs before his acting career. While appearing on M*A*S*H, he also wrote five episodes and directed four episodes of the series. Besides being a writer and a director, he has also been an executive producer and a producer in both television and film.

Even before he was well-known, Farrell has been an activist for many political and social causes. He has worked with Human Rights Watch, supports long-term efforts to end homelessness and hunger, opposes capital punishment and he is pro-choice. He has long been deeply involved in organized labor, particularly as an officer of the Screen Actors' Guild, and has publicly supported actions including the Los Angeles grocery workers' union strike of 2003-2004.

He was the voice of Jonathan Kent on Superman (1996) series with Shelley Fabares, his wife, playing Martha Kent. He has guest starred in Murder, She Wrote, Justice League, Matlock, Mannix, and many others.

He married actress Judy Farrell in 1963. They were divorced in 1983. They have two children, Michael and Erin.

On December 31, 1984, he married actress Shelley Fabares.

On April 6, 2006 Farrell was scheduled to participate in a play about the Guantanamo Bay detainment camp in front of members of Congress.