Mary Wilson did a brave thing Tuesday night. She finally left the Supremes.

One might never have guessed it, though, from the medley of the group's old hits she packed into the disco debut of her new act. After 19 years, Wilson knows that the Supremes' name still carries a rare cachet, and she isn't ready yet to let her audiences forget who she once was.

"This is the last bit of the Supremes," she said during rehearsals last week. "I have to make that final statement -- then it's on to the songs from my new album."

Her new identity was a long time coming. Eleven years after Florence Ballard quit the group, later to die penniless in Detroit, and nine years after Diana Ross abandoned the trio to become a solo star, the last of the original Supremes was making a comeback at 35 as Mary Wilson. It had not been easy.

"We were all pretty bitter when Diana left," she conceded. "But I never get too depressed. Besides, it was never directed towards Diana. It was towards Motown (Records)."

Wilson felt that Motown reduced its commitment to the group after Ross left and, after nearly 10 years, her relationship with Berry Gordy, the Motown mogul who gave them their first contract, has only recently returned to what she describes as "pretty good."

There was no sudden disaster when Ross departed, despite initial fears. In fact, the Supremes, with Cindy Birdsong and Jean Terrell had a few more hits such as "Stoned Love" in the early '70s before gradually disappearing from the top-40 charts and concentrating on friendly European audiences.

It was Wilson who kept the reconstructed group together during these years, breaking in one new Supreme after another, and watching their currency drop until they finally gave their farewell performance in London two years ago.

Still, Wilson couldn't completely sever the ties and toured briefly as "The Supremes' Mary Wilson" until she and Mowtown Records locked horns in a lawsuit over her use of the name and the terms of her new contract as a soloist.

"Mowtown didn't give me what I thought I should get in the contract -- promotion, options on albums, things like that," she explained. "They treated me like I was a newcomer, not someone who had helped build the company."

The case was settled out of court this year, and she opened Tuesday night at the Manhattan discotheque "New York, New York" simply as Mary Wilson. She now has a new contract, a new album (on Motown) and a five-year marriage to Pedro Ferrer, her manager.

Wilson is heavier than when she sang to British royalty more than 10 years ago. Her face is fuller now, a bit like Gladys Knight. She is trying to take off more of the weight she gained when she had her third child recently. She said that it's hard.

But Tuesday night she showed that she is still a stunning woman, with eyes that are big and liquid and lovely, and flawless skin.

Backed by a 12-piece band and two women in roles not unlike the ones she and Ballard once had, Wilson appeard on stage Tuesday with a strong voice, a lot of guts, and two costume changes. Diana Ross sat in the wildly partisan audience -- including Joel Grey, Andy Warhol and a host of record-industry officials -- and Ross held Wilson's young daughter in her lap as she cheered her old colleague on. If the evening was no great critical success, it was full of emotion.

Wilson's medleys were a little unsettling. It was painful for some to to hear rapid-fire snatches of songs like "Stop in the Name of Love," "Come See About Me," and "Baby Love." They wanted more of each and the memories that went with them. Perhaps Wilson did too.

"Am I scared?" she said five days earlier. "You better believe I am. I was used to singing 'oohs' and 'babys.' Now there are words. I had to learn all over again."

Wilson had to learn more than words. She had to learn how to act like a lead singer, use an ego that until recently seemed as quiet as a car on blocks.

Her decision to end the Supremes marks the final step out of the considerable shadow of Diana Ross, a born lead singer, whom Wilson concedes is "a tough act to follow." It was also the mark of a mature woman who knows something about risks.

"If I'm going to fight, I might as well fight for myself instead of a group that has had its day," she explained. "And now is not the time for me to relax."

The original Supremes, in their years, were backed up by the great songwriting team of Holland-Dozier-Holland and brilliantly packaged by Berry Gordy Jr., the legendary force behind Detroit's Motown Record Co. By 1966, Wilson said, that combination of talent had transformed the Supremes from a group of women who began singing together in the housing projects of Detroit into a national pop institution. The Supremes sold 12 gold albums by the time Ross left in 1970 and played around the world.

"I've been on the road 10 months a year since 1964, and I love it," Wilson said. "We lived well. We met the queen of England and Lawrence Olivier and people like that. We used to go out and buy two or three fur coats for the fun of it."

Wilson won't be traveling that way when she takes her troupe to Europe for three months after a six-day stint in New York.

"We have been performing in Europe for the past five or six years," she explained. "It's hard in this country to perform at the level I am accustomed to. In Europe, we didn't need a hit record. The Supremes have always been more popular in England than in the U.S."

Her own music must flourish on both continents if she is to succeed, but Wilson is optimistic: "I know the formula for hit records," she said. "I can get one -- maybe not the first time out -- but I can get one."