The former couple - who were married for five years before splitting in 2005-– have remained friendly since their divorce, and have grown closer after Brad separated from his second wife Angelina Jolie in 2016.
The 55-year-old arrived at the star-studded bash at the Sunset Tower Hotel in West Hollywood on Saturday, February 09 wearing a flat cap and quickly made his way inside.
Earlier in the evening, Jennifer, who turns 50 on Monday, stunned in a strappy black jumpsuit and heels as she entered the historic hotel, which is reportedly her favourite Hollywood hotspot.
Katy Perry, boyfriend Orlando Bloom, George Clooney and Amal Clooney, Reese Witherspoon, Ellen DeGeneres, Kate Hudson, Demi Moore and Jennifer Meyer were also on hand to celebrate the milestone with the former "Friends" star.
Brad, who has been reportedly dating Charlize Theron since January, arrived solo at the bash.
Jennifer, who split from Justin Theroux in 2017 after two years of marriage, recently opened up about her past relationships, and insisted she would never stay in a troubled marriage for fear of being alone.
"My marriages, they've been very successful, in personal opinion. And when they came to an end, it was a choice that was made because we chose to be happy, and sometimes happiness doesn't exist within that arrangement anymore," she explained.
"To stay in a marriage based on fear feels like you're doing your one life a disservice."