The happy couple were pictured arriving at Cafe Habana, where they joined the likes of Cindy Crawford, her husband Rande Gerber, who owns the restaurant, and U2's The Edge.
The celebrity group gathered on the back patio, where they toasted George and Amal with tequila and wine.
Smiling Amal could be seen showing off her impressive seven-carat diamond engagement ring as she chatted with her husband-to-be's close friends.
There was even a bit of karaoke; Bono got up and sang during the party, as did supermodel Cindy, who reportedly duetted with her daughter Kaia.
George, 53, proposed to British human rights lawyer Amal, 36, in late April, after seven months of dating.
The future Mr and Mrs Clooney recently returned from a holiday in Cabo, Mexico, where they celebrated a friend's wedding. "You can tell these two are in love," an onlooker said.