She began her career being credited as Lisa Munn. However, since 2006, she has been using the name Olivia Munn professionally. Also since 2006, Munn has been one of the faces of the cable network G4, hosting a number of shows for the network, the most prominent being Attack of the Show!, with co-host Kevin Pereira.
Munn moved back to the United States, settling in Los Angeles, California to pursue an acting career. In 2004, she interned at Fox Sports Net and worked as a sideline reporter for college football and women's basketball. She has gone on to say that she disliked the experience, explaining "I was trying to be something I wasn’t, and that made me really uncomfortable on live TV." Munn had also gained a small role in the straight to video horror film Scarecrow Gone Wild. She also stars in the music video Hello Tomorrow by the band Zebrahead.
In late 2005, Munn began her portrayal of Milly Acuna, a teen surfer, over two seasons of the TV drama Beyond the Break on The N network. She enjoys surfing and continues to practice the sport. She originally auditioned for the part of Kai, but the producers wanted a "local girl". She also appeared in the film The Road to Canyon Lake.
Munn is a successful model and has booked campaigns for Nike, Pepsi and Neutrogena. She appeared on Fall 2006 cover of Foam magazine in September, and Men's Edge magazine in August, and was featured in a pictorial in Complex magazine in November 2006. In February 2007, she appeared as "Babe of the Month" in a non-nude pictorial in Playboy magazine. Munn also appeared in the July/August 2007 issue of Men's Health magazine. In September 2007, she was featured in the magazine, Italian Vanity Fair for their "Hot Young Hollywood" Issue.
Munn will make her debut in a large Hollywood film in the upcoming Rob Schneider movie Big Stan. She will play Schneider's character's receptionist Maria. Munn also has a significant role in the 2008 horror film Insanitarium, in which she plays a nurse at an insane asylum.