The birthday girl documented her crazy day on Instagram.
Sofia kicked things off with a glam session before getting on a private jet with Kylie, 22, and some other friends.
The ladies first Sin City outing was at a poolside party at Encore Beach Club where the birthday girl partied in a sexy bikini.
Her boyfriend Scott, 36, cozied next to her at the wet n' wild bash.
The Flip it Like Disick star got his ladylove a black Aston Martin car for her milestone birthday.
Other events during Sofia's big day included a classy dinner at Cipriani's followed by a night at XS nightclub.
Many celebs that wished Sofia a happy 21st including model Winnie Harlow and nightclub owner Dave Grutman.