'NCIS' Star Mark Harmon Reveals the Secret to His Long-Lasting Success: 'You Earn Luck'

After four decades in the spotlight, Mark Harmon has mastered the keys to his successful career — and credits them to one of the most influential people in his life: his father.

“My dad taught me, if it’s not true, don’t worry about it,” the NCIS star tells PEOPLE in this week’s cover story.

“I come from working stock, and I’m proud of that. You have to get up every day and just try hard,” says Harmon, 67. “That’s what I still wake up willing to do: try my hardest.”

Harmon was born and raised with two older sisters — Kristin, who died last year at age 72, and Kelly, 70, who lives in Los Angeles — in Burbank by his late dad, football and broadcasting icon Tom Harmon, and his wife, artist Elyse Knox.

Tom was a college star who went on to play two seasons in the NFL and Harmon followed in his footsteps, playing quarterback and eventually starting for UCLA.

“I’m a practice guy, a repetition guy,” he says. “You can tell me I can’t do something, but I’m likely to take that as incentive to prove you wrong. I’m a tough guy, I’m a jock, so you can yell at me all day long, and that won’t hurt me — I’m just going to try to get better. In this business I’ve used that to my advantage.”

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Although Harmon’s dad died suddenly at age 70 when the star was just 38, the longtime actor continues to follow the profound advice that Tom imparted.

“Work your ass off, and don’t let the effort show,” he says. “Also: Treat people the same, not based on who they are or what they are. Show them common kindness. Don’t be colored by their fame or profession. I don’t get all excited by people blowing smoke up my ass. You have to stay honest.”

While Harmon is widely recognized for his starring role as Leroy Jethro Gibbs in CBS’ NCIS — currently the highest-rated drama on network TV — for the past 16 years, the Golden Globe and Emmy-nominated actor has been a household name for decades.

In 1986, Harmon, who shares two sons — Sean, 30, and Ty, 26 — with his wife of 31 years, actress Pam Dawber, was named PEOPLE’s second-ever Sexiest Man Alive.

“Oh, it isn’t that I hate it, really,” Harmon says with a sly grin about the the honorable title. “It’s just that I’m not about to take it seriously. And let’s just say I think maybe a few too many people have.”

But fame doesn’t faze the the notoriously private actor.

Rather, it’s honed a deep sense of gratitude in the father of two — and a pushed him to be better at his craft each day.

“This choice, this career, requires you to be all in. Your job is to be prepared for the opportunity — it will come, you just won’t know when,” he says.

“So until then, all you can do is invest in yourself,” the star advises. “I’ve always worked hard, I’ve always tried to pay attention to the little things — but here’s the real secret: there are no little things.”

And while some may be struck by luck alone and propelled to stardom, Harmon believes his long-lasting career in Hollywood required more.

“I’m a lucky guy,” he says, “but you earn luck.”