The birthday girl started off the evening's festivities at Catch LA. Stepping out of a car with pal Hailey Baldwin, Kendall showed off her killer legs in a skin-tight, lace-up black bodysuit. She paired the look with strappy heels and an olive green Charlotte Simone fur scarf.
Kendall then slipped into a slinky, silver sequin backless mini dress with a plunging neckline to hit up West Hollywood hot spot Delilah.
At that second soiree, she was presented with a giant birthday cake by
Kris and Kim that had a throwback childhood shot of Kendall on it.
The model was joined by her famous family at both locations. Her parents, Kris and Caitlyn Jenner, both came out, as did her sisters, Kylie Jenner, Kourtney and Khloe Kardashian. Kourtney's ex Scott Disick, Kylie's boyfriend Tyga, and Kris' beau Corey Gamble, were all there as well, as were celebrity pals Baldwin, Cindy Crawford, Kaia Gerber, Jonathan Cheban, Melanie Griffith, Kate Mara, and Gabrielle Union.
Kim Kardashian West and Kanye West were also in attendance, ET can confirm. Kim, who has been keeping a low-profile since her traumatic Paris robbery, did not appear to be seen in any social media postings.
Noticeably missing from the soiree, however, was Kendall's older brother, Rob Kardashian, and his fiancée, Blac Chyna, who are expecting their daughter any day now.
Kourtney, who stepped out in a binding silver dress, posted several selfies of her outfit and more pics with her sisters in honor of Kendall's big day.
Baldwin also took to Instagram to share a shot with the birthday girl in a leopard-printed photo booth, writing, "Happy 21st birthday to my babe. I love you lots, happy to be here with you tonight @kendalljenner"
The biggest surprise of the night came when Kendall was escorted outside with her many guests only to find a brand new Rolls Royce, a gift from her mom, waiting for her with a large peach-colored bow on top. The excited supermodel gasped and giggled as she saw her new ride for the first time.
Kendall has been all about celebrations this week. Check out her fun Halloween festivities in the video below.