Father, Tom Harmon, played for the University of Michigan Wolverines and won the 1940 Heisman Trophy emblematic of the best player in the nation.
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 TV show while growing up was "Combat!" (1962).
QB of the 1972-1973 UCLA Bruins. Teammate of Randy Gaschler.
His first acting gig was playing a gorilla on "Ozzie's Girls" (1973).
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.
He broke an eardrum while shooting Beyond the Poseidon Adventure (1979).
He had a four-year relationship with "Flamingo Road" (1980) co-star, Cristina Raines, which ended in 1984. (They met while filming 1978's "Centennial" (1978) miniseries).
Studied pre-med in college before transferring to communications. He later played doctors on "St. Elsewhere" (1982) and "Chicago Hope" (1994).
His Airstream trailer can be seen in the flashbacks in Hiatus part 1 &2. In season 2 episode "Black Water" his trailer and his 1984 Porsche can be seen as the villain drives off.
Chosen as People Magazine's Sexiest Man Alive. 
One of the most notorious characters that Mark played was serial killer Ted Bundy in 1986's TV movie, The Deliberate Stranger (1986) (TV).
Mark has broken his shoulder twice. Once during the filming of the movie, Summer School (1987), and while filming a Coors commercial.
Mark and Pam Dawber married, exactly one year after they met, on March 21, 1987. They met March 21, 1986.
Harmon and Pam Dawber have two children: Sean Harmon (born on 25 April 1988) and Ty Harmon (born on 25 June 1992).
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. .
His oldest son, Sean Harmon, plays a young Gibbs on "NCIS: Naval Criminal Investigative Service" (2003).
On the set of "NCIS: Naval Criminal Investigative Service" (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.
Mark received a call from the producers during the summer between seasons 3 and 4 of "NCIS: Naval Criminal Investigative Service" (2003) telling him to stop shaving, because his character Jethro Gibbs was to have a more gruff look, along with a mustache.
He has appeared nude in multiple works including: 1988 "The Presidio" (region 2), 1989 "Sweet Bird of Youth", 1991 "Fourth Story", and "NCIS: Naval Criminal Investigative Service" (2003), Season 3 Paris flashbacks.
His mother, Elyse Knox (who was married to the legendary Tom Harmon), passed away on February 15, 2012, at the age of 94.