The future Mr. and Mrs. Clooney were joined by some of their famous friends, including U2 frontman Bono, the restaurant's owner Rande Gerber and his supermodel wife, Cindy Crawford.
Sources told E! News the low-key celebration was well underway around 7 p.m.
Clooney looked relaxed and was dressed in a casual blue shirt with jeans. His human rights attorney wife-to-be wore a green T-shirt with floral shorts
One eyewitness said the restaurant was very busy because of the Mother's Day crowd.
"People got a show they didn't expect and other [restaurant patrons] said it was the best Mother's Day ever," the source exclusively told E! News.