The Iron Man 2 beauty reveals she had never suffered from spotty skin until she moved from Japan, where she was raised, back to her native Oklahoma at the age of 16 and experienced a nasty outbreak of acne.
She says, "I had this horrible outbreak and I never had acne before. On my skin, on my forehead, it was covered in acne and I didn't know what to do. I'm a new kid in school and so I felt really self-conscious and didn't really lift my head up until my cousin mentioned it.
"I hadn't even heard of Proactiv, and she said, 'You should just try this.' I used it and within like a week all of it was gone."
However, the acne recently returned while she was shooting upcoming crime thriller Deliver Us From Evil, much to Munn's horror.
She tells People.com, "I'm doing a movie with Eric Bana where I play his wife, and I've got this massive cystic acne on my neck, my cheek and on my forehead. And he's so charming and good-looking, and thankfully the first time I met him was at night. But I thought, 'No!' That's when I really went full force and tried and everything. Nothing was working."
Munn turned to treatments like acupuncture and steroid injections from a dermatologist, but nothing helped until her agent suggested she try a new Proactiv+ formula.
She explains, "I guess he had also noticed my pimples. I thought, 'Will it work? I'm an adult now!' Within two weeks it was completely cleared up and I haven't had a breakout again."
The actress jumped at the chance to join the likes of Justin Bieber, Adam Levine and Katy Perry and become a spokesmodel for the skincare brand, because she knows first-hand that it really works: "It's so crazy because I have this whole story with it. It was nice. It's something that's very easy to get behind because I know what it did for me in my life."