Photo: Rebecca Greenfield

Mellody Hobson has worked at Ariel Investments ever since she started as a college intern in 1997. Maybe that's why she roams far and wide to hone her leadership skills and collect big ideas.

Besides overseeing operations at Chicago-based Ariel, the mutual fund company where she became president at 31, Hobson is on a bunch of big boards: DreamWorks Animation (DWA), Estée Lauder (EL), Groupon (GRPN), and Starbucks (SBUX).

Last week at Fortune's annual Most Powerful Women dinner in New York City, Hobson, now 43, told editor at large Pattie Sellers what she's learned inside the boardrooms.

Sitting on the Starbucks board taught Hobson the importance of a CEO's personality, she said. "When I get fearful, I think, 'What would Howard [Schultz] do' And he would say, 'Mellody, you're thinking too small.'"

"At DreamWorks, it's Jeffrey Katzenberg's hard work," Hobson told the audience of 150 MPW leaders. "The first time I met him, it was his third breakfast. And it was only, like, 7:30 a.m."

At Estée Lauder, "it's the remarkable fabric of people that truly believe in the successful family story--and family company."

While these three Fortune 500 giants retain a startup mentality that helps keep them successful, four-year-old Groupon bears plenty of criticism for its management and financial reporting practices--which Hobson is trying to help with. "Dealing with a startup, when you're in the boardroom, you're a genius," she said, laughing. Who helped her decide to join the Groupon board? Warren Buffett, no less. The Berkshire Hathaway (BRKA) CEO urged her to accept the post because she'd learn a lot, he said--since Groupon is upending the way companies reach consumers.

Hobson is a self-made success. She grew up in inner-city Chicago, the youngest of six children raised by a single mother. "I didn't grow up in a family where the stock market was discussed; I didn't know anything about it. Nothing," she said. As she told senior editor Jennifer Reingold for a 2008 Fortune profile, Hobson "happened into the investment industry" by meeting Ariel founder John Rogers on her way to Princeton. Rogers was a recruiter for the University.

A couple of principles have guided Hobson ever since:

"Be the labor, big or small. Do it well or not at all," her mother used to tell her.

And, from a John Lennon song, Beautiful Boy: "Life is what happens to you while you're busy making other plans."

Lacking a plan, Hobson simply worked to be excellent--knowing that excellence would lead to good things. Determined to master something new each decade, she became an avid runner in her 20s (she has completed five marathons) and a superstar swimmer in her 30s. (She says she can swim three or four miles without stopping.)

Now Hobson is deciding on her goal for her 40s. Should she master the piano or a foreign language? She's leaning toward the former. Her mother, 83, started taking piano lessons two years ago--and gets better each time Mellody hears her play.

To learn more lessons from the boardrooms, watch this interview that Fortune Executive Editor Stephanie Mehta did with Hobson last week before the MPW dinner: