The actress complained that she didn't get a chance to sing much when she last hosted the show 14 years ago and realised it was because she has a terrible voice - but that didn't stop her having a go at the top of Saturday's broadcast.
The cast complimented Theron on her acting skills, charity efforts and sex appeal, but tried to shut her up every time she started to sing.
The Monster star also showed off her comedy chops on the show, portraying a fired drama teacher on fake teen programme Girlfriends Talk Show, a sex-starved accent-removal coach, a desperate cat fanatic at an adopt-a-thon, and a naive teenager whose dream beach date turns messy when she's covered in guts from an exploding dead whale. The actress stayed in her bloody beach costume to introduce Snl musical guests The Black Keys.
She also shot a comedy skit on the streets of New York, pretending to be an overweight tourist.