Inventing a new way to make money on the Internet, and competing with behemoths like Google didn't come easily to AppNexus founder and CEO Brian O'Kelley.

He had to get fired-more than once.

"I've gotten fired a few times, and every time it was the same reason," O'Kelley told CNBC. "I saw this huge incredible opportunity to do something that would change the world and the people I worked for said, 'hey, that's not realistic.'"

O'Kelley's rise to a billion-dollar valuation came with a few bumps. He was fired as Chief Technology Officer at Right Media, which was automating advertising networks, the day after the company was sold to Yahoo for $850 million in 2007.

But the AppNexus chief is in fairly good company. The late Steve Jobs was once famously fired from Apple, the company he eventually turned into a global brand with his knack for technology innovation.

O'Kelley now works for himself, with some taking to calling him the king of ad technology at one of New York's most valuable start-ups.

"I feel like I have to live every day at work as if it could be my last," O'Kelley said. "I have to take the risks that allow us to potentially fail because it's the only way to learn."