Harmon gained recognition for portraying Secret Service special agent Simon Donovan in The West Wing, receiving a 2002 Emmy Award nomination for his acting in the four-episode story arc.
Harmon was cast in a similar role a year later: Donald P. Bellisario—the creator of both JAG and NCIS for CBS—had seen Harmon in The West Wing and decided to cast him in NCIS. Harmon's character of NCIS special agent Leroy Jethro Gibbs was first introduced in a guest starring role in two episodes of JAG. Since 2003, Harmon has starred in NCIS as the same character.
Harmon was born in Burbank, California, the youngest of three children and the only son. His parents were Heisman Trophy–winning football player and broadcaster Tom Harmon and actress and artist Elyse Knox (née Elsie Lillian Kornbrath). Harmon has two older sisters, actress and painter Kristin Nelson, who was divorced from the late singer Rick Nelson, and actress-model Kelly Harmon, who was once married to car magnate John DeLorean. His maternal grandparents were Austrian immigrants.
After graduating from high school at Harvard School, Harmon completed a two-year associate degree at Pierce College in Los Angeles, then transferred to the University of California, Los Angeles, where he was the starting quarterback for the UCLA Bruins football team in 1972 and 1973.
After his sophomore season at Pierce, Harmon had a number of offers from major college football programs, and he ultimately chose UCLA over Oklahoma. The Sooners finished second in the nation in 1971, while the Bruins were a pre-season top-20 selection and stumbled to a 2–7–1 record and were last in the Pac-8.
During his very first game for UCLA, he engineered a stunning upset of the two-time defending national champion, Nebraska Cornhuskers. The Bruins were an 18-point home underdog to the top-ranked Huskers, but won 20–17 with a late field goal by Efren Herrera under the lights in the L.A. Coliseum. In his senior year in 1973, Harmon received the National Football Foundation Award for All-Round Excellence. During his two years as quarterback in coach Pepper Rodgers's wishbone offense, UCLA compiled a 17–5 record (.773). Harmon graduated cum laude from UCLA in 1974 with a B.A. in Communications.
He was inducted into the Pierce College Athletic Hall of Fame among its first class of members in 2010.
After college, Harmon considered pursuing a career in advertising or law. Harmon started his career in business as a merchandising director, but soon decided to switch to acting. He spent much of his career portraying law enforcement and medical personnel. One of his first national TV appearances (other than as an athlete) was in a commercial for Kellogg's Product 19 cereal with his father, Tom Harmon, its longstanding TV spokesman. Thanks to his sister, Kristen's in-laws, Ozzie Nelson and Harriet Nelson, he landed his first job as an actor in an episode of Ozzie's Girls. This was followed by guest roles in episodes of Adam-12, Police Woman, and Emergency! in mid-1975. He also performed in "905-Wild", a backdoor pilot episode for a series about two L.A. County Animal Control Officers which did not sell. Producer/creator Jack Webb, who was the packager of both series, later cast Harmon in Sam, a short-lived 1978 series about an LAPD officer and his K-9 partner. Before this, Harmon received an Emmy nomination for Outstanding Supporting Actor in a Miniseries or a Movie for his performance as Robert Dunlap in the TV movie Eleanor and Franklin: The White House Years. In 1978, he appeared in three episodes of the mini-series, Centennial, as Captain John MacIntosh, an honorable Union cavalry officer.
During the mid to late 1970s, Harmon made guest appearances on TV series' such as Laverne & Shirley, Delvecchio, The Hardy Boys/Nancy Drew Mysteries and had supporting roles in the feature films Comes a Horseman (1978) and Beyond the Poseidon Adventure (1979). He then landed a co-starring role on the 1979 action series 240-Robert as Deputy Dwayne Thibideaux. The series centered around the missions of the Los Angeles County Sheriff's Department Emergency Services Detail, but was also short-lived.
In 1980, Harmon gained a regular role in the prime time soap opera Flamingo Road, in which he played Fielding Carlisle, the husband of Morgan Fairchild's character. Despite initially good ratings, the series was canceled after two seasons. Following its cancellation, he landed the role of Dr. Robert Caldwell on the prestigious NBC Emmy-winning series St. Elsewhere in 1983. Harmon appeared in the show for almost three seasons before leaving in early 1986 when his character contracted HIV through unprotected intercourse, one of the first instances where a major recurring television character contracted the virus (the character's subsequent off-screen death from AIDS would be mentioned two years later). In the mid-1980s, Harmon also became the spokesperson for Coors Regular beer, appearing in television commercials for them.
Harmon's career reached several other high points in 1986. In January, he was named People magazine's Sexiest Man Alive. Following his departure from St. Elsewhere in February, he played the lead in the TV movies Prince of Bel Air, co-starring with Kirstie Alley, and The Deliberate Stranger, in which he portrayed the real-life law student turned cross-country serial killer Ted Bundy. With his career blossoming, he played a role in the 1986 theatrical film Let's Get Harry and the lead role in the 1987 comedy Summer School, again co-starring with Kirstie Alley and alongside future JAG/NCIS alum Patrick Labyorteaux. Returning briefly to episodic television in 1987, Harmon had a limited engagement on the series Moonlighting, playing Cybill Shepherd's love interest Sam Crawford for four episodes. He then starred in the 1987 TV movie After the Promise. In 1988, he co-starred with Sean Connery and Meg Ryan in the 1988 feature film The Presidio, and also opposite Jodie Foster in the film Stealing Home. Despite several high-profile roles, Harmon's film career never gathered momentum and, after a muted reception to his 1989 comedy Worth Winning, he returned to television, appearing in various television movies.
Harmon's next regular television role would be as Chicago police detective Dickie Cobb for two seasons (1991–1993) on the NBC series Reasonable Doubts. In 1993, he appeared in one episode in the role of a rodeo clown on the CBS comedy/western series Harts of the West with future castmate Sean Murray, who plays McGee on NCIS.
In 1995, Harmon starred in the ABC series Charlie Grace, in which he portrayed a private investigator. The series lasted only one season, after which he returned to ensemble medical shows on the series Chicago Hope, in which he played Dr. Jack McNeil from 1996 to 2000. He also portrayed astronaut Wally Schirra in one episode of the 1998 mini-series From the Earth to the Moon.
In May 2002, he portrayed Secret Service special agent Simon Donovan on The West Wing in a four-episode story arc. The role gained him his second Emmy Award nomination, exactly 25 years after his first nomination. Harmon appeared in a guest starring role in two episodes of JAG in April 2003, which introduced the character of NCIS agent Leroy Jethro Gibbs; since 2003, Harmon has starred as Gibbs in the CBS drama NCIS, a role which earned him three nominations at the People's Choice Awards. During his time on the show, he was reunited with three of his former Chicago Hope co-stars, Rocky Carroll, Lauren Holly, and Jayne Brook. Since 2008, he has also been a producer/executive producer.
In 2003, Harmon had a supporting role in the remake of the comedy film Freaky Friday.
Harmon has also starred in several stage productions in Los Angeles and Toronto. At the Cast Theatre in Los Angeles he performed in Wrestlers and The Wager. In the late eighties he was part of the cast of the Canadian premier of Key Exchange. Several productions of Love Letters provided him the opportunity to play alongside his wife Pam Dawber.
Harmon received the 2,482nd star of the Hollywood Walk of Fame on October 1, 2012.
In 2014, Harmon started a production company called Wings Productions to produce NCIS: New Orleans.
Harmon directed two episodes of Chicago Hope in 1999 and 2000. He also directed two episodes of Boston Public in 2002.
Harmon is the son of football star Tom Harmon and actress Elyse Knox. His sisters are Kelly, an actress-model, and Kristin, an actress-turned-painter.
Harmon worked as a carpenter before making a success of his acting career. On NCIS, his carpentry skills are alluded to through his character's hobby of building boats in his basement.
Harmon has been married to actress Pam Dawber since March 21, 1987. The couple have two sons; Sean Thomas Harmon (born April 25, 1988, who played a young Gibbs in five NCIS episodes), and Ty Christian Harmon (born June 25, 1992). They maintain a low profile and the couple rarely appear in public with their children. Harmon was the brother-in-law of Ricky Nelson and John DeLorean and is the uncle of actress Tracy Nelson and singers Matthew and Gunnar Nelson of the pop duo Nelson.
In 1987, Harmon filed for custody of his nephew Sam on the grounds that his sister, Kristin Nelson, was incapable of good parenting. Sam's psychiatrist testified that the thirteen-year-old boy depicted his mother as a dragon and complained about her mood swings and how she prevented him from being with his siblings. Harmon later dropped the custody bid.
In 1988, Harmon was part owner of a minor league baseball team in San Bernardino, California, the San Bernardino Spirit, which spawned Ken Griffey, Jr. Harmon used the team and their home field, Fiscalini Field, for the opening and closing scenes of a baseball movie he was starring in, Stealing Home.
In 1996, Harmon saved a teenage boy involved in a car accident outside his Brentwood home. Harmon used a sledgehammer from his garage to break the window of his burning car, then pulled the boy from the flames.