The couple, who broke up in August, reconnected last weekend in New York, and were then spotted together in L.A. celebrating Mayer’s 31st birthday on Wednesday night.
The bash was held at the Grand Havana Room, a members-only restaurant and cigar club.
After stopping by a party for pal David Arquette’s new clothing line, called Propr, Aniston, 39, met up with the group, staying past midnight to ring in Mayer’s birthday.
“They seemed to be having a great time,” a source tells PEOPLE. “They were sitting next to each other. They’re a very good-looking couple and seemed happy together.”
It was their second outing in L.A. this week, following a dinner with friends the night before at the Sunset Towers.