Harmon was born and raised in Southern California; the son of actress Elyse Knox and Heisman Trophy winner Tom Harmon, a 1954 College Football Hall of Fame inductee from Michigan. He attended The Harvard School (now known as Harvard-Westlake) in Los Angeles, playing football, baseball and rugby. On the gridiron, he mostly took the field as a running back and safety, only appearing in four games at quarterback. He broke his elbow as a junior, and did not play varsity football as a senior.
Not recruited out of high school, Harmon headed to Pierce Junior College in Woodland Hills, California, and he quarterbacked the team to a 7-2 record in 1971, earning All-America laurels. His performance earned him multiple scholarship offers, including Oklahoma in an effort led by Barry Switzer, the offensive coordinator at the time and a future College Football Hall of Fame coach, to recruit him. Harmon opted to stay in his hometown of Los Angeles, playing for UCLA head coach Pepper Rodgers and assistant coaches Homer Smith, Lynn Stiles and Terry Donahue, also a future College Football Hall of Fame coach.
Playing alongside future College Football Hall of Fame inductees Randy Cross and John Sciarra, Harmon helped orchestrate a UCLA turnaround, quarterbacking the Bruins, which had finished 2-7-1 at eighth-place in the Pac-8 in 1971, to a combined 17-5 record in 1972 and 1973. In his first game ever as a Bruin, which opened the 1972 season, Harmon led an underdog UCLA to a dramatic 20-17 win against two-time defending national champion Nebraska, snapping the Huskers' 32-game-unbeaten streak.
A Wishbone-T quarterback who could run, pass, fake and mix plays, Harmon rushed for more yards and touchdowns than he did passing, amassing 1,504 yards rushing and 14 touchdowns versus passing for 845 yards and 9 touchdowns during his tenure in Westwood. The offensive coordinator Homer Smith's wishbone offense forced Harmon to make multiple decisions in very short time periods, distributing the ball to running backs Kermit Johnson and James McAlister. The combination created the top running game in the nation in 1973, and UCLA set school records for total yards gained (4,403), average yards per game (400) and rushing touchdowns (56).
A Communications major who aspired to become a doctor, Harmon excelled in the UCLA classrooms, carrying a 3.45 GPA and graduating cum laude. His accomplishments earned him Second Team CoSIDA Academic All-America honors in 1973 as well as an NFF National Scholar-Athlete Award, which led to his trip to New York City where he was honored at the NFF Annual Awards Dinner the same night that John Wayne accepted the NFF Gold Medal.
"In today's society, the scholar-athlete is indeed a rare breed," Harmon said in responding for the NFF Scholar-Athlete Class in 1973. "Not only does he excel on the field, but he competes in the classroom as well…. As we gather here tonight to pay our respects to the men who made the great American game of football what it is today, we hope that one day in the future some of us from the Class of '74 might be fortunate enough to carry on the great tradition that has been passed down by the distinguished men in this room. If we do, it is because our universities gave us the chance and the game of football has given us the principles."
After UCLA, Harmon declined professional football offers to instead pursue acting. He worked in advertising, as a shoe company rep and as a carpenter between acting gigs and appearing in Coors beer commercials. His hard work eventually paid off with a big break on NBC's St. Elsewhere and the leading role of Dr. Robert Caldwell. His success continued on NBC's police drama Reasonable Doubts starring as detective Dickey Cobb and CBS's Chicago Hope where he appeared as Dr. Jack McNeil.
He also had memorable arcs on the hit shows Moonlighting and The West Wing before landing the lead role of Leroy Jethro Gibbs, a special agent with the Naval Criminal Investigative Service, on CBS' global favorite NCIS series, which has become part of television history, approaching its 400th episode druing its 17th season. The most-watched-scripted show on American television today and consistently ranked among the five highest-rated TV shows each year, NCIS is a TV juggernaut, attracting more than 15 million viewers each week throughout most of its run.
In 2011, Harmon became an executive producer on NCIS, and in 2014 an idea he co-developed became the spinoff NCIS: New Orleans which premiered on CBS with Harmon as an executive producer alongside Gary Glasberg. His big-screen credits include Freaky Friday, Wyatt Earp, The Presidio, Summer School and Stealing Home. He has worked with Elizabeth Taylor, Michael Caine, Sean Connery, Jamie Lee Curtis, Jodie Foster, Allison Janney, Karl Malden, Patricia Arquette and Denzel Washington among countless other Hollywood notables.
Harmon has received numerous accolades and award nominations during his career, including being honored with a star on the Hollywood Walk of Fame on Oct. 1, 2012, but he has always remained humble with an appreciation for the efforts of others. Quietly giving back, Harmon's charitable work includes Saving Bristol Bay, Stand Up To Cancer, Ronald McDonald House Charities, Kids Wish Network, Clothes Off Our Back, Entertainment Industry Foundation, Oklahoma Kidz Charities Foundation, Oklahoma City Indian Clinic and The Children's Center OKC.
From his time as a quarterback at UCLA until now, as an executive producer and star of the CBS hit series NCIS, Harmon has always treated teammates and production crews with familial respect and loyalty.
"I look at the show as a team," Harmon said during a previous interview. "I've always been a team guy. I'm not in [acting] for the personal part of this, and I wasn't as an athlete either. It's about the work and we all work together."
In 2019, the NFF presented Harmon with the NFF Gold Medal, the organization’s highest honor in recognition of his exceptional accomplishments, unblemished reputation and for reflecting the values of amateur football.
