Harmon and his wife, Pam Dawber, have two sons: Sean Harmon (born on 25 April 1988) and Ty Harmon (born on 25 June 1992).
Chosen as People Magazine's second annual "Sexiest Man Alive," between inaugural winner Mel Gibson, who won for 1985, and third year winner Harry Hamlin, who won for 1987. 
His godfather, Forest Evashevski, is in the National Football Foundation's College Football Hall of Fame for coaching.
Risked his life to save two teenage boys who were involved in a car accident outside of his home. Harmon used a sledgehammer from his garage to break the window out of their car, then pulled them free so they wouldn't be burned to death, while his wife, Pam Dawber, called 911. He made every effort to downplay his role in saving their lives. .
He graduated cum laude from UCLA, 1974, with a degree in communications. Also played quarterback for UCLA Bruins, 1972-1973, winning National Football Foundation Award for all-around excellence.
Worked as a carpenter before he hit it big, foretelling the carpentry and wooden boat-building interest and skills of Leroy Jethro Gibbs, the character he plays on NCIS (2003).
No relation to Angie Harmon, although both have starred in a series also starring Sasha Alexander..
Not the grandson of former U.S. Secretary of the Navy William Franklin "Frank" Knox. Mark's mother, former actress Elyse Knox, was born Elsie Lillian Kornbrath to Frederick and Elizabeth Kornbrath in Hartford, Connecticut; she was not the daughter of Navy Secretary Knox, despite several incorrect sources that have said she was.
His legendary father, Tom Harmon, played halfback for the University of Michigan Wolverines (1938-1940), winning the 1940 Heisman Trophy, emblematic of the best player in the nation, for his efforts.
Mark was quarterback of the 1972 and 1973 UCLA Bruins football teams, and was a teammate of center Randy Gaschler; linebacker and defensive end Fulton Kuykendall; fellow quarterback (and future Philadelphia Eagles pro defensive back) John Sciarra; defensive end (and future Minnesota Vikings pro linebacker) Fred McNeill; cornerback (and future Pittsburgh Steeler and Detroit Lions pro cornerback and safety) Jimmy Allen; and center (and future Seattle Seahawks and New England Patriots pro center) Art Kuehn; among others.
Studied pre-med in college before transferring to communications. He later played doctors on St. Elsewhere (1982) and Chicago Hope (1994).
Early in his career, he was known for being the face of Coors beer, for doing more than 20 Coors beer commercials in the early 1980s.
His oldest son, Sean Harmon, portrayed a young Gibbs on NCIS (2003).
Was also a lifeguard before hitting it big. His work as a lifeguard was a summer job at Laguna Beach, California, where his parents and the Nelsons both had beach houses. His father used to wake him up early so he could run for an hour on the beach before he had to report for work. He has credited this practice as conditioning him for fall football practice. Another job he had early in his acting career was working with Mike Walden as the color man on tape-delayed UCLA telecasts on KTLA. On one of those telecasts. Harmon discussed his then-upcoming series, Sam (1978).
Was a national shoe representative for Adidas in the 1970s.
His first paid acting gig was playing a gorilla on Ozzie's Girls (1973). The show's titular star and producer, Ozzie Nelson, was his relative by marriage.
Mark has broken his shoulder twice. Once during the filming of the movie Summer School (1987), and another incident while filming a Coors beer commercial.
Mark studied American Sign Language for his various television roles with The Sign Company.
Mark has an abducted left knee from his football career.
Mark has been known to wear shirts that were sent to him by his fans.
When he was 16 years old, at his after-school job delivering flowers, he found a dozen roses addressed to a girl who was dating a friend of his. But the roses weren't from his friend. Mark threw away the card and substituted one with his friend's name on it.
In his first UCLA game, Mark led the Bruins to a 20-17 underdog win over Nebraska, snapping the two-time defending national champion's 32-game winning streak.
Mark received a call from the producers during the summer between seasons 3 and 4 of NCIS (2003) telling him to stop shaving, because his character Jethro Gibbs was to have a more gruff look, along with a mustache.
Was considered for the role of Dan Gallagher in Fatal Attraction (1987).
Younger brother of Kelly Harmon and Kristin Harmon.
Uncle of Tracy Nelson, Matthew Nelson, Gunnar Nelson, and Sam Nelson.
Had two famous (or infamous) brothers-in-law: car magnate John DeLorean, who was convicted in federal court of drug charges related to raising money to keep his car company afloat with drug proceeds, and Rock and Roll Hall of Fame inductee Ricky Nelson, who died from severe burns in a DC-3 plane crash that was, at one time, thought to have been caused by Ricky was using the gas heater on the plane to ignite drugs. The drug theory was later discounted.
His mother, Elyse Knox-who was married to the legendary Tom Harmon)-passed away on February 15, 2012, at age 94.
Has appeared nude in multiple works, including The Presidio (1988) (region 2), Sweet Bird of Youth (1989), Fourth Story (1991), and NCIS (2003)'s season 3 Paris flashbacks.
After his very successful college football career at UCLA, he was offered the opportunity to turn professional with a contract to play for the NFL New England Patriots. It wasn't an easy decision but he turned it down. He didn't think he was good enough, but mainly thought that college was about getting an education and making something out of it, as opposed to using college to build a career as a jock.
Had a four-year relationship with Flamingo Road (1980) co-star Cristina Raines, which ended in 1984. (They met while filming the miniseries Centennial (1978)).
His favorite actors are Gary Cooper, Buster Keaton, and William S. Hart. The license plates on his 1956 Dodge pickup and 1984 Porsche read YUHP and UH-HUH-both common Gary Cooper sayings.
Favorite television series while growing up was Combat! (1962).
He broke an eardrum while shooting Beyond the Poseidon Adventure (1979).
He and his wife, Pam Dawber, are gun-control advocates, despite wearing a holster in his job.
His Airstream trailer can be seen in NCIS (2003) flashback scenes in "Hiatus: Parts 1 and 2." In the season 2 episode "Black Water," his trailer and his 1984 Porsche can be seen in the back lot of the TV studio as Gibbs and DiNozzo arrive to interview Celina Lockhart.
On the set of NCIS (2003), Mark uses a 1972 Airstream trailer that he restored himself as a dressing room. The trailer has red-and-white checkered floors, and an 8-track player.
One of the most notorious characters that Mark played was serial killer Ted Bundy in the television movie The Deliberate Stranger (1986).
Mark received the nickname 'Quarterback' because his dad was an announcer for the football games that Mark played in when he was younger. His dad did not want to call Mark by name, so instead he called him Quarterback.
Mark and Pam Dawber married on March 21, 1987, exactly one year after they met,. They met on March 21, 1986.
When he was younger, he ran 45 miles a week.
Was paid $2,000,000 for doing the Coors commercials back in the 1980s.
Back in the day, he had a bearded collie named Ryan and a cat named Arnold.
Almost chose to play football for Oklahoma. Later he decided to play for the Bruins because their program was on a downswing and he wanted to help turn it around.
Before they were married, Mark Harmon and Pam Dawber once went on a double date with Julianne Phillips and Bruce Springsteen.
Was a marathon runner.
Gary Cooper is his acting idol, due to the respect Mark has for Cooper's acting philosophy.
First television appearance was with his father in a commercial for Kellogg's Product 19 cereal.
He loves dogs. He once had an Australian shepherd, a golden retriever, and a bearded collie.
Has a 1951 Woody that he and his dad restored.
Was a marathon runner and knows how to play the guitar.
Bought his first car when he was 13: a 1928 Ford.
Received a star on the Hollywood Walk of Fame in front of 6253 Hollywood Boulevard in Hollywood, California on October 1, 2012.
Was a big runner after college; used to run 60 to 70 miles a week.
He now does pilates to keep in shape.
When he was younger, he bore a strong resemblance to Ricky Nelson, which subsequently led him to be misidentified in newspapers as Nelson's brother when they were actually only brothers-in-law.
In 2011, he was voted the fourth-most-popular actor on prime-time television.
He was cast as Leroy Jethro Gibbs on NCIS (2003) after series creator/producer Donald P. Bellisario saw his commanding performance as a Secret Service agent on The West Wing (1999).
Mark's father was of three-quarters Irish ancestry, with the rest being French and German. The family surname was changed from Hermann to Harmon several generations back. Mark's maternal grandparents were Austrian (from Vienna).
He broke his collarbone during a spring practice football scrimmage in 1973, at UCLA.
First actor to play Ted Bundy.
He has played the same character, Leroy Jethro Gibbs, in five different series: JAG (1995), NCIS (2003), NCIS: Los Angeles (2009), Family Guy (2012) and NCIS: New Orleans (2014).
Mark and Sean Murray starred in the episode "The Right Stuff" of Harts Of The West in 1993. Ten years later they were together again in NCIS.
Born on exactly the same date as Michael Gray (of "Shazam!" fame).