Best known for playing Sweet Dee on the FX series It's Always Sunny in Philadelphia alongside Charlie Day, Glenn Howerton, Rob McElhenney, and Danny DeVito. She also appeared on The Drew Carey Show and Curb Your Enthusiasm. She later starred on FOX's The Mick.
A horrific bicycle accident left her with a shaved head at the start of her freshman year of high school, and she forced herself to be more outgoing to make up for it.
After spending time with The Groundings comedy troupe in Los Angeles, she earned several Hollywood auditions, including one for the show Punk'd.
She was born in Tigard, Oregon, and her father, Don Olsen, was a former publisher of the Portland Tribune. On September 27, 2008, she married It's Always Sunny in Philadelphia co-star Rob McElhenney. She and Rob had a son named Axel in 2010 and a second son, named Leo, in 2012.
She appeared in an episode of Miss Match opposite Alicia Silverstone in 2003.