Chavez, 49, is the firm’s CIO, but that doesn’t begin to tell his story. He’s part tech pooh-bah, part guru, and the newest member of Goldman’s 29-member executive committee. He’s also an example of how people can reinvent themselves at Goldman — and in life.

Arriving at Goldman (GS) in 1993 after earning his Ph.D. in medical information science at Stanford, Chavez worked as a quant in the energy business — but left four years later, claiming he needed a new challenge. That wasn’t the real reason.

“The real reason is I made a big change in my life,” says Chavez. “I decided to stop drinking.” Seeking a change of venue (people in recovery call it “pulling a geographic”), he moved to Credit Suisse, but says he knew he’d made a terrible mistake. After a few years he left the industry, founded and sold a startup, bought a beach house, and retired to Fire Island, N.Y.

Soon after that, he got a call from now-president Gary Cohn, then co-head of global securities. “I heard you sold your company — congratulations,” Chavez recalls him saying. “I heard you retired. That’s ridiculous. I was just calling to share with you that you’re coming back.” When Chavez told him he was burned out on commodities, Cohn suggested investment banking. Chavez didn’t know the first thing about banking, but Cohn was insistent.

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Chavez was reluctant. But after some soul-searching at a silent monastery in New Mexico, he says he had an epiphany. “It’s the only time in my life that I would say there was a clear message from the universe,” he says. It might have been that he wanted out of his task for the day — cleaning toilets — but Chavez says he realized that he wanted to effect transformation in a global way and that returning to Goldman (No. 45 on Fortune’s 100 Best Companies to Work For) was the way to do it. He returned, made partner a year later, and soon was named global co-chief operating officer of equities. “After that I became pretty fearless,” he says.

So last year when the firm told him it needed him to become CIO, he was game. Goldman had just suffered a coding glitch that caused it to lose tens of millions on errant trades. Chavez, with his tech background and stature within the firm, was the ideal person. So far, so good, he says. Young people often ask Chavez what it takes to do well. He has an answer he’s honed over the years: If Goldman Sachs is your top priority in life, he says, it probably won’t work (“That’s wonderful, but I don’t want to hang out with you,” he says); but if it’s your fourth, that probably won’t cut it either. He advises them to make a list of their “short and sacred list of priorities.” For him, he says, Goldman is a solid three. “Not one, not two,” he says. “And definitely not four.”

This story is from the February 3, 2014 issue of Fortune.