Born February 6, 1939 in Minnesota, Mike Farrell was two years old when his family moved to LA; his father, a carpenter, had just gotten a lucrative movie-studio job. Exposed to showbiz from an early age, Farrell began acting in high school plays, hoping to pursue the theatre as a career. He was forced to put his thespic urges on the back burner during his hitch with the U.S. Marines, but upon being discharged he attended drama courses at Los Angeles City College and UCLA, and also studied at the Jeff Corey Workshop. He made his professional debut in a 1961 stage production of Rain, then spent several years playing bits in such films as Captain Newman MD (1963), The Graduate (1967) and Targets (1968). His first real break came in 1968, when he was cast as architect Scott Banning on the NBC daytime drama Days of Our Lives. Two years later, he put his John Hancock on a contract with Universal, playing supporting roles in such prime-times series as The Interns (1969) and Man and the City (1971).
Unhappy with the type of roles offered him by his studio, Farrell asked for and received his release in 1975 when the opportunity came to audition for the popular sitcom M*A*S*H. Wayne Rogers had just left that top-rated series, leaving an opening in the category of "Hawkeye's Best Friend." Farrell read for the assignment, hit it off immediately with M*A*S*H leading-man Alan Alda (something Rogers had never been able to do), and was cast as wise-cracking army surgeon B. J. Hunnicutt, a role he'd fill until the series' final episode in 1983. Like Alda, Farrell directed several M*A*S*H episodes; also like Alda, he was a dedicated political and social activist, devoted to such causes as gay rights and prevention of child and spousal abuse. Since M*A*S*H's demise, Farrell has chosen to cut down on his acting appearances, preferring to direct; in addition to his series-TV work as director, he has also helmed the 1988 TV movie Run Till You Fall. In 1988, he co-produced the critically acclaimed theatrical feature Dominick and Eugene with Marvin Minoff , and reunited with Minoff to co-produce the 1998 drama Patch Adams.
Though Farrell has guest starred in a number of television shows throughout the 1980s, 90s and 2000s (among them include Murder, She Wrote, Justice League, Matlock, and Desparate Housewives), his most significant television role since M.A.S.H was perhaps that of veterinarian Jim Hansen, whom he portrayed in the NBC drama Providence (1999 - 2002).