She played several minor roles in various television comedies including Becky on Curb Your Enthusiam (1999-2024) before she was cast as Dee Reynolds on It's Always Sunny.

Since then, she has gone on to appear in several comedy films, including Leap Year (2010), The Heat (2013), Vacation (2015), Finding Dory (2016), Arizona (2018) and Champions (2023).

[6] She then went on to receive two more Emmy nominations for Outstanding Guest Actress in a Comedy Series in 2022 and 2024 for her role as DJ in Hacks.

[15] At age twelve, Olson was in a serious bicycle accident involving a vehicle, resulting in a fractured skull requiring reconstructive surgery.

In 2000, she made her debut television appearance on HBO's Curb Your Enthusiasm in the recurring role of Cheryl Hines's sister.

Olson also made several guest appearances in films and television series including The Drew Carey Show, The Jamie Kennedy Experiment, Out of Practice, Miss Match, Family Guy, and Punk'd.

[18] In 2004, Olson was cast as Deandra "Sweet Dee" Reynolds in the FX sitcom It's Always Sunny in Philadelphia (2005-present), one of the then-four (later five) main characters on the show.

In the 2013 film, The Heat, she appeared as a Bulgarian drug addict who engages in an exchange of cultural perspectives (and insults) with Melissa McCarthy's character.