The 29-year-old E! star added a darling collage of six snapshots to her Instagram account on Saturday, Dec. 14 to commemorate his special day.
"My happiness is this little man! Happy birthday Mase!!" the brunette beauty captioned the sweet pic.
The tribute to the 4-year-old shows photos of the duo throughout the years. The precious pair is sporting sunglasses and similar YOLO hats, as well as making ducky faces for the camera with Kylie Jenner.
Khloé added a second snapshot, a faceless pic posing with the birthday boy in matching star pants.
"Mase and KoKo," she added along with the photo.
It appears that Khloé is keeping busy and spending time with her family the day after she filed for divorce from Lamar Odom.
Mason, meanwhile, has been celebrating his birthday with a Disney-filled week. Proud mama Kourtney Kardashian Instagrammed several pictures of their Disneyland festivities—including a ride on It's a Small World, a visit to the Haunted Mansion, checking out some holiday décor and watching the fireworks over Princess Aurora's castle! The group also hopped on the merry-go-round—with Mason riding his own white horse while mom and Penelope, 1, shared another—and embarked on Tomorrowland's Finding Nemo Submarine Voyage.
Kourtney and Scott Disick took their boy to see Disney On Ice presents Rockin' Ever After on Thursday, Dec. 12.
Mason and his dad enjoyed some father-son time and went out for breakfast on Friday, Dec. 13.
Happy birthday, Mason!