The Fall Out Boy bassist started with dinner in a private room at Mr. Chow, accompanied by his bandmates, soon to be father-in-law, Joe Simpson, pal Joel Madden and about 10 other male friends.
Clad in a black hoodie, Wentz smiled from ear to ear as he left the Beverly Hills restaurant. He was congratulated on the way out by a restaurant patron, who shook his hand. Simpson, who seemed in great spirits, clapped his hands onto Wentz's shoulders and said, "This is the man."
After the group left shortly before 11 p.m., they headed to the Hollywood Roosevelt Hotel, where they partied in the Marilyn Monroe suite that overlooks the Tropicana Bar and Roosevelt pool.
"It was Pete's bachelor party!" a source tells PEOPLE.
Ashlee's big sister, Jessica, was at the hotel's lounge, Teddy's, where Wentz briefly stopped in to check out the weekly Hennessy Jazz Night.
"Jessica was there with her friends. They stopped in and said it was awesome," says the source. "She was really enthusiastic about it."
Meanwhile, the bride-to-be, Ashlee, held her own bachelorette party at Kiki De Montparnasse, a luxury lingerie and intimacy products store.
Reporting by JENNIFER GARCIA, ABBY STERN and MARK GRAY