Harmon and Pam Dawber have two children: Sean Harmon (born on 25 April 1988) and Ty Harmon (born on 25 June 1992).
Chosen as People Magazine's Sexiest Man Alive. 
His godfather Forest Evashevski is in the National Football Foundation Hall of Fame.
Risked his life to save two teenage boys who were involved in a car accident outside of his home. Harmon used a sledge hammer 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. .
Graduated cum laude from UCLA, 1974, with degree in communications. Also played quarterback for UCLA Bruins, 1972-1973 winning National Football Foundation Award for all-round excellence.
Worked as a carpenter before he hit it big.
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 Franklinn (Frank) Knox; his mother, former actress Elyse Knox--born Elsie Lillian Kornbrath to Frederick and Elizabeth Kornbrath in Hartford, Connecticut--was not Knox's daughter, despite several sources that say she was.
Father, Tom Harmon, played for the University of Michigan Wolverines and won the 1940 Heisman Trophy emblematic of the best player in the nation.
QB of the 1972-1973 UCLA Bruins. Teammate of Randy Gaschler.
Studied pre-med in college before transferring to communications. He later played doctors on St. Elsewhere (1982) and Chicago Hope (1994).
Was the second winner of People magazine's "Sexiest Man Alive" poll.
Did over 20 Coors beer commercials in the 80s.
His oldest son, Sean Harmon, plays 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, CA , where his parents and the Nelsons each had beach houses.His dad 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 while in his early acting career,was working with Mike Walden as the color man on tape delayed UCLA telecasts on KTLA. On one of these telecasts. he discussed his upcoming series, Sam.
Was a national shoe representative for Adidas in the 1970s.
His first acting gig was playing a gorilla on Ozzie's Girls (1973).
Mark has broken his shoulder twice. Once during the filming of the movie Summer School (1987), and while filming a Coors commercial.
Mark studied 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 which have been 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 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, Sam Nelson.
Had two famous brothers-in-law: car magnate John DeLorean and Rock and Roll Hall of Fame inductee Ricky Nelson.
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), Season 3 Paris flashbacks.
Had the opportunity to go Pro and play for the Patriots. It wasn't an easy decision but he turned it down. Didn't think he was good enough, but mainly thought that college was about getting an education and making something out of it.
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 were 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 the flashbacks in "Hiatus: Parts 1 and 2". In 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, exactly one year after they met, on March 21, 1987. They met 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, their program was on the downswing and 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 idol, respects his 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, 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, it was a 1928 Ford.
Received a Star on the Hollywood Walk of Fame at 6253 Hollywood Boulevard in Hollywood, California on October 1, 2012.
Was a big runner after college. Used to run 60-70 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 in fact they were actually only brothers-in-law.
In 2011, he was voted the fourth most popular actor on prime time television.
Was cast as Gibbs on NCIS (2003) after Donald 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 had been 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 three different series: "JAG" (1995)_, NCIS (2003) and NCIS: New Orleans (2014).