Mark Harmon in NCIS

Take a look back at NCIS star Mark Harmon's early career – he looks so different!

The actor first appeared on screens nearly 50 years ago

Eve Crosbie

Mark Harmon has built up quite the fanbase since he began playing agent Gibbs on the hit CBS series NCIS, but the TV star was no stranger to success before the drama began.

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In fact, the 69-year old actor first appeared on screens nearly 50 years ago when he was just 22 years old.

In 1973, Mark bagged a part on the sitcom Ozzie's Girl, thanks in some part to his sister Kristin who was married to creator Ozzie Nelson's son. While he only appeared on one episode of the show, it opened the door for a number of other roles on primetime shows and it wasn't long before Mark was regularly appearing on screens.

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WATCH: Mark Harmon discusses being named 'Sexiest Man Alive' on SNL

In fact, just ten years later, in 1983, Mark was cast on the popular medical drama St. Elsewhere which turned him into an overnight TV sensation. He appeared on the series as Dr Robert Caldwell for three seasons before making his exit in 1986 - the same year he was named Sexiest Man Alive by People magazine.

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Several other TV shows followed, including the soap opera Flamingo Road, and The Deliberate Stranger, in which he portrayed the real-life serial killer Ted Bundy. He briefly ventured into the world of cinema, appearing alongside Sean Connery and Meg Ryan in the film The Presidio and also opposite Jodie Foster in Stealing Home. In 1989, he even was cast as the lead in rom-com Worth Winning, but after the film received something of a muted reception from audiences, he returned to television.

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Mark in soap opera Flamingo Road

In 2002, he appeared in a four-episode story arc in political drama, The West Wing. The part not only gained him his second Emmy nomination but led to him landing his role on NCIS. After the episode aired, creator Donald P. Bellisario invited Mark to guest star on his show JAG, where he was introduced as the character Leroy Jethro Gibbs. A spin-off all about his character leading a fictional team of Naval Criminal Investigative Service special agents based in Washington, D.C. was then commissioned.

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Mark got his first break when he starred in St. Elsewhere

MORE: Where are the stars who left NCIS now?

To date, Mark has appeared on every single episode of the show across 18 seasons, making him one of the longest-serving American TV stars. While NCIS certainly wouldn't be the same without him, fans have been sent into a tailspin over worrying reports that he could be departing the series after the upcoming 19th season. Speculation about the actor's future on the show began earlier this year amid reports that he's hoping to take a step back.

A question remains about how much involvement Mark will have in the new series. Insiders close to the CBS drama report that he will only make a few appearances throughout the show's latest season, although this has not yet been confirmed.

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