What do most professional basketball players do after retiring?

“A lot of guys get on the golf course,” says Kevin Willis, whose 23 years in the NBA included a 2003 championship with the San Antonio Spurs. “A lot try to get into some type of business venture, but when you don’t really have the experience, or you’re not knowledgeable, it doesn’t go well.” That’s an understatement. (Ask Scottie Pippen or Latrell Sprewell, both of whom blew their basketball bucks post-retirement.)

Yet Willis, 49, a still-muscular 7-footer, made the transition so smoothly it was almost as though the NBA was merely a long, lucrative interlude in the life of his business. In a way, it was. He’d known since 1988, when he and Michigan State classmate Ralph Walker opened their first Willis & Walker big and tall clothing store in Atlanta, that he would focus on fashion once he left basketball. Willis built the business even as he prospered in the league, playing an amazing 21 seasons before leaving at 45, in 2007. During his tenure, he says, “There wasn’t a day when I wasn’t on the phone at least once, or doing something physically, that pertained to my business.”

Originally Willis & Walker focused on custom suits, priced around $1,200. They then expanded into jeans, with great success: NBA players would come in when their team was in Atlanta and buy as many as eight pairs at just under $200 a pop. Willis & Walker clothing has been sold in Rochester Big & Tall, which has 27 U.S. stores, and in Stanley Korshak, a Dallas department store. It almost made it to Nordstrom (JWN), but Willis shot an air ball; his factory put in five-inch zippers on all the jeans rather than the seven inches he favors.

Now, however, Willis & Walker is trying to become a global lifestyle brand, thanks to a new equity partnership with Marcraft Apparel Group, which also licenses Tommy Hilfiger and Marc Ecko. Marcraft will take over manufacturing and help build connections with retailers. “The reason Kevin appealed to us wasn’t because he’s famous or played in the NBA,” says Gary Brody, co-president of Marcraft, “but because he’s a really smart designer.” Pre-Marcraft, the store brought in some $1 million in sales. Now Willis thinks the group can become a mainstream brand. Already, Willis & Walker has one new deal in the can with Men’s Wearhouse (MW).

It won’t be easy. After all, if a rich athlete wants a suit, why would he choose a Willis & Walker over an Ermenegildo Zegna? Brody contends it’s because Willis knows customers’ needs better; Willis adds that taller-size suits by other labels are boxy and hang like a curtain.

In September, Willis presented the first-ever Willis & Walker runway show during New York’s Fashion Week. Models trotted out in slim-fitting suits and Japanese denim with suede jackets. Guests included NBA alum Charles Oakley (who Willis estimates owns more than 30 Willis & Walker suits) and actor Quinton Aaron, who starred in The Blind Side. Can Willis & Walker become the go-to brand not just for NBA stars but all big men? Says Catherine Moellering, head of trend-forecasting firm Tobe: “These are consumers who have been marginalized. So the potential is huge.” Huge indeed.

This article is from the November 7, 2011 issue of Fortune.