Britney, 26, made a surprise appearance at a fundraiser for Jenny McCarthy’s autism charity Generation Rescue in LA on Saturday.
As guests like Jim Carrey and Charlie Sheen and wife Brooke Mueller made the rounds, Britney sipped on red wine.
Jim’s daughter Jane also belted out a few Frank Sinatra and Jackson 5 tunes.
Britney “definitely looked happy,” a witness told Us magazine. “She was clapping and smiling a lot.”
It was Britney’ first public outing since reaching a custody agreement with ex Kevin Federline last week.
While Kevin gets sole custody, Britney gets three visits a week from Sean Preston, 2 1/2, and Jayden James, 1 1/2, including two overnights.
If all goes well, she could have her boys roughly 40 percent of the time, her attorney, Laura Wasser, has said.
Laura Wasser told the Associated Press the “difference” in Britney “from the beginning of this year to now is astonishing.”
Britney is spending her summer recording a new album.