Mark Harmon: "You Earn Luck" - The Secret to NCIS star Long-Lasting Success

After 40 years in the spotlight, Mark Harmon has mastered the secrets to his successful career — and he attributes 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 grew up in Burbank, California, with his parents, former football and broadcasting star Tom Harmon, and his wife Elyse Knox, and two older sisters, Kristin (who died at the age of 72) and Kelly (age 70, resides in Los Angeles).

Tom was a star college player who played in the NFL for two seasons and Harmon followed suit, starting as a quarterback 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.”

Even though Harmon’s father passed away at the age of 70 when he was only 38 years old, the veteran actor still follows the wise advice that Tom gave him.

“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 most well-known for his role as Leroy Jethro Gibbs in CBS’s “NCIS” – the network’s highest rated drama for the last 16 years – the Golden Globe- and Emmy-winning actor has been in the public eye 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.

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.”

While some people are born lucky and starry-eyed, Harmon believes he needed more to have a long-lasting Hollywood career.

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