A bevy of entertainment's elite showed up to mark the occasion at upscale Sunset Tower on Sunset Boulevard, and a rip roaring good time seemed to be had by all - most obviously by Reese Witherspoon.
The Legally Blonde actress took a tumble down the stairs as she left the bash, gripping onto the hand rail for support before being helped to her feet by hotel staff.
The 42-year-old looked stunning in a black mini-dress and matching coat.
She teamed the look with a pair of vertiginous platform stilettos, gold drop earrings and hot pink lipstick.
Seeing the funny side of her slip, the mother-of-three broke into a wide grin as she stepped into a waiting vehicle outside the venue.
Reese has been a close friend and colleague of Jen's for many years, having played Rachel's spoiled little sister Jill on Friends in 2000.
The famous blondes are currently filming a new TV show together, and Reese once described Jen as "an awesome friend and a woman who lets you in".
A glittering array of industry heavyweights came together for Jennifer's milestone birthday, including her Friends co-stars Courteney Cox and Lisa Kudrow, Gwyneth Paltrow, Sandra Bullock, Kate Hudson and Robert Downey Jr.
Jen's ex-husband Brad Pitt made a surprise appearance, while ex Justin Theroux was notably absent as he partied the night away at a Fashion Week event in New York City instead.