Power Players

This is the No. 1 piece of advice Kevin O'Leary would give his younger self — and it would have saved him a lot of money

"Shark Tank" star Kevin O'Leary is a successful businessman and financier, but like most people, he's made his fair share of mistakes, including gambling away his money in his early 20s in Las Vegas.

But the top advice 60-year-old O'Leary would give to his 20-year-old self is pretty simple: "Don't buy all that crap you don't need," O'Leary tells CNBC Make It.

"I think about all the crap I bought that I never used. I mean it's amazing," he says. "All the shoes, all the pants, all of the shirts that I never wore after the first time I bought them. What a waste of money.

"I could have had that money making 7 or 8 percent in perpetuity" by investing it, O'Leary says.

Kevin O'Leary
Courtesy of Kevin O'Leary
Kevin O'Leary

It's the same reason O'Leary refuses to pay even $2.50 for a cup of coffee; he knows if he makes that coffee at home for pennies, he can put the savings to better use.

O'Leary frequently preaches the importance of saving and investing, and recommends that young people harness the magic of compounding returns.

"When you're 21 years old, or 20 or 18 or 19 and you start putting aside 10 percent of what you make, [you could have] over $1,000,000 by the time you're 65," O'Leary previously told CNBC Make It. "If no one else is going to worry about your retirement, I want you to worry about it."

O'Leary has added a string of zeros to the number of dollars in his bank account since he was 20, so he is now personally willing to spend on investment pieces that will last for years and on his favorite splurge, $150 underwear.

But that doesn't mean he's wasteful. O'Leary tells CNBC Make It he only wears four pairs of shoes — two pairs of Prada shoes, a pair of flip flops and a pair of sneakers for working out.

"I started off with probably 100 pairs of shoes. It's like the Imelda Marcos problem," O'Leary says, referring to the corrupt former first lady of the Philippines, who was famously known for her excessive shoe collection. "You don't need that problem."

"So if you're 20 — listen to what Mr. Wonderful is saying here — you don't need 50 shirts," he says. You only need a handful of essentials, which he says includes six t-shirts, three pairs of jeans and three baseball caps.

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Disclosure: CNBC owns the exclusive off-network cable rights to ABC's "Shark Tank."