The awards, part of the Hollywood Film Festival, were handed out at The Beverly Hilton Hotel and recipients included Clooney, who was honoured with the Actor of the Year prize for his roles in The Ides of March and The Descendants.
Mulligan landed the Hollywood Supporting Actress Award for her role in Shame, and Williams was honoured with the Hollywood Actress Award for My Week with Marilyn, in which she plays Marilyn Monroe.
The actress used her acceptance speech to reflect on her role as the late screen icon, telling the crowd, ""It seems to me that all that Marilyn ever wanted was to be taken seriously as an actress. She studied, she trained harder than me, and she never got the recognition that she deserved and craved. I feel very lucky tonight to experience some of that.""
The stars of new movie The Help were handed the Hollywood Ensemble Cast Award, while Glenn Close was chosen as the 2011 Career Achievement Award honouree.