The guest of honor's parents, who broke up in 2015 and co-parent their three kids, were photographed arriving separately at the venue, the Offsunset lounge in West Hollywood. Scott walked in with their youngest child, son Reign Disick, 8,
"Mazel Tov Mason!!!" the Good American founder wrote on Instagram. "The girls and I attempted to take some pics."
Mason, whose dad Scott is Jewish, is the eldest of the momager's 12 grandchildren. The now-teen is the first junior member of the famous family to have a bar mitzvah, a Jewish coming-of-age event.
Mason turned 13 on Dec. 14. "The day has come where you're taller then [sic] me now," Kim wrote on her Instagram Story that day, sharing a rare photo of her eldest nephew, who has shied away from the spotlight in recent years. "I just love who you are and I'm just so proud of you Mason. Welcome to your teenage years. Happy Birthday."
Scott wrote on his own Instagram Story at the time, "Happy birthday 2 my better half. Love you mason."
Mason shares a birthday with his little brother Reign. On her Instagram, Kourtney shared a sweet throwback photo of her sons.
"December 14th… one of the best days of my life," Kourtney wrote Dec. 14. "Twice and forever. my birthday boys."
On Dec. 17, Scott shared a pic of Reign on his Instagram Story. "The man," he said. "One down one to go."
See photos of Kardashian-Jenner family members at Mason's bar mitzvah party: