Olson was a member of the improvisational group The Groundlings. She toured with the USO to Bosnia, Kosovo and Norway. She made several guest appearances in films and television series such as Curb Your Enthusiasm, The Drew Carey Show,The Jamie Kennedy Experiment, Out of Practice, Miss Match, Family Guy, and Punk'd. She had a minor role in the film Coyote Ugly.

In 2004, she was cast as Deandra "Sweet Dee" Reynolds in the FX sitcom It's Always Sunny in Philadelphia. She also voiced Ethel in the first season of the Comedy Central animated series Brickleberry. In the 2013 film The Heat she appeared as a Bulgarian druggie who engages in an exchange of cultural perspectives and insults with Melissa McCarthy's character.

She played a recurring role as Hartley Underwood, the "one-armed" neighbor in the FX series The Riches. In June 2016, the Human Rights Campaign released a video in tribute to the victims of the 2016 Orlando nightclub shooting; in the video, Olson and others told the stories of the people killed there.

Olson stars in the Fox sitcom, The Mick, which premiered in January 2017. She also serves as an executive producer for the series.

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