The This Is Us actor, 42, attended producer Jennifer Klein’s annual holiday party at her Los Angeles home, sources tell PEOPLE exclusively.
“Justin was one of the last of the guests to leave. He was there until at least midnight,” an onlooker tells PEOPLE. “He was in great spirits and was being super social. He worked the room and caught up with guests like Chace Crawford, Topher Grace and Jennifer Morrison.”
Hartley wasn’t the only newly single star to hit the party. Christina Hendricks — who announced her split from Geoffrey Arend in October — was the belle of the ball at the festive celebration, where guests sipped on Don Julio cocktails, Good Day coffee and snacked on Shake Shack burgers and a Chipotle taco bar.
“She was the happiest she’s ever looked and was dancing up a storm,” says the onlooker. Hendricks, who wore a floor-length black dress with white ruffles, was seen partying next to Machine Gun Kelly and also caught up with another newly single actor — Dave Annable — near the dance floor.
Welcomed by Justin Wine and Le Grand Courtage bubbly, celebs entered into Klein’s packed-to-the-brim home and dug into a custom See’s Candies bar. Other festive treats included Sprinkles layer cakes, brownies and cookies, barkTHINS, Brookside chocolate, Jeni’s ice cream, Lily’s Sweets, Pipcorn and Sidecar Doughnuts.
Last week, multiple sources confirmed to PEOPLE that Stause had moved out of Hartley’s house and into a new home.
In documents obtained by PEOPLE, Hartley cited irreconcilable differences as the reason for their divorce, and listed the date of their separation as July 8, 2019 — despite the fact that the pair attended the Emmy Awards together on Sept. 22, the Veuve Clicquot Polo Match in Los Angeles on Oct. 5, and a Hollywood Reporter-sponsored pre-Golden Globes party on Nov. 14.
“Yes, they’d had some problems, but she certainly never thought he would just go and suddenly file for divorce, and not even try to save things,” one source previously told PEOPLE.
But an additional insider pointed out that Stause is not the only one upset that their relationship has ended.
“Justin is obviously very disappointed that his marriage didn’t work,” the insider said, adding that the couple’s work schedules often kept them apart.
The duo tied the knot in 2017 in a rustic and romantic ceremony at Calamigos Ranch in Malibu, California, exchanging vows in front of 75 of their closest friends and family members.