The son of a former Heisman Trophy winner and a 1940s film actress, Mark Harmon took naturally to the gridiron and to acting, although what he really wanted to be as a youth was a doctor. After two years quarterbacking a junior-college team in Los Angeles, Harmon transferred to UCLA and successfully led the Bruins during the 1972 and '73 seasons, with an intent to attend medical school after graduation. But that all changed when Harmon had the good fortune to meet TV legend Ozzie Nelson (Harmon's sister Kristin married his son, singing star Rick Nelson) who offered him his first acting gig, a walk-on role on the syndicated Ozzie's Girls in 1973. Acting was now Harmon's passion. Other bit TV parts followed, but it was his Emmy-nominated turn as a physically impaired veteran in the TV movie Eleanor and Franklin: The White House Years that made Hollywood take notice. He went on to tackle a string of guest spots, miniseries and short-lived series, including the primetime soap Flamingo Road, before becoming a sex symbol and the doctor he'd always wanted to be on the medical drama St. Elsewhere. Harmon exited the series in 1986 after his character contracted AIDS, a controversial subject at the time, and found success in features (The Presidio, Stealing Home), in commercials (an ad for Coors beer) and in a wide array of TV movies, most notably as serial killer Ted Bundy in The Deliberate Stranger and as the title character in the TV biopic Dillinger. Harmon's personal life was also clicking; he married Mork & Mindy star Pam Dawber in 1987 (they have two sons), and over the next decade he went on to headline the series Reasonable Doubts and Chicago Hope. But Harmon's biggest role was still to come. After an Emmy-nominated appearance on The West Wing, Harmon was approached to play Special Agent Jethro Gibbs on the military drama JAG. Soon after, Gibbs was the lead character on the JAG smash spin-off NCIS. The show's success has allowed the family-oriented Harmon, who once worked as a carpenter to make ends meet, to drastically reduce his appearances in other projects and only take occasional roles (Chasing Liberty, Weather Girl) that interest him.
- The son of Heisman Trophy winner Tom Harmon, he quarterbacked the UCLA Bruins during the 1972 and '73 seasons and received the National Collegiate Football Foundation Award for All Around Excellence.
- Worked briefly as a salesman for a tennis-shoe company and as a carpenter.
- Got an early acting break when Jack Webb offered him a part on Adam-12 after first suggesting that the long-haired, mustachioed Harmon adopt a more clean-cut appearance.
- Is the brother of actress Kristin Nelson, who is the mother of actress Tracy Nelson and of twins Gunnar and Matthew of the singing duo Nelson.
- Was named People magazine's Sexiest Man Alive in 1996.
- Saved the lives of two teens in 1996, using a sledgehammer to break into a burning car in which the victims were trapped.
- 1993, Golden Globe — Best Performance by an Actor in a Television Series - Drama: Nominee
- 1992, Golden Globe — Best Performance by an Actor in a Television Series - Drama: Nominee
- 1996, Screen Actors Guild Awards — Outstanding Performance by an Ensemble in a Drama Series: Nominee
- 1977, Emmy — Outstanding Performance by a Supporting Actor in a Comedy or Drama Special: Nominee
- 2002, Emmy — Outstanding Guest Actor in a Drama Series: Nominee
- 1997, Screen Actors Guild Awards — Outstanding Performance by an Ensemble in a Drama Series: Nominee
- 2017, People's Choice Awards — Favorite TV Crime Drama Actor: Winner
- 1988, Golden Globe — Best Performance by an Actor in a Miniseries or Motion Picture Made for Television: Nominee
- 1987, Golden Globe — Best Performance by an Actor in a Miniseries or Motion Picture Made for Television: Nominee
- Elyse Knox — Mother
- Ty Christian Harmon — Son
- Gunnar Nelson — Nephew
- Sean Thomas Harmon — Son
- Kristin Harmon — Sister
- Tracy Nelson — Niece
- Kelly Harmon — Sister
- Matthew Nelson — Nephew
- Pam Dawber — Wife
- Tom Harmon — Father
- Attended Pierce College, Woodland Hills, California, United States; University of California, Los Angeles, California, United States (BA in Communications, 1974)