She played minor roles in various television comedies, including Becky in the HBO sitcom Curb Your Enthusiam (2000–2020), before she was cast in It's Always Sunny.

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

In 2000, Olson made her debut television appearance in the HBO comedy series Curb Your Enthusiasm as the recurring role of Becky, Cheryl Hines's sister.

Olson went on to make guest appearances in several television series, including The Drew Carey Show (2002–2004), Punk'd (2003), Miss Match (2003), George Lopez (2004), and Out of Practice (2006).

[18] Olson received wider recognition when she 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 series.

In 2007, Olson played a recurring role as Hartley Underwood, the "one-armed" neighbor in the FX drama series The Riches.

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

In 2016, Olson provided the voice of Destiny, a nearsighted whale shark, in Disney's animated sequel film Finding Dory.

In 2023, Olson had a voice role in the Netflix animated series Agent Elvis and starred in the sports comedy-drama film Champions.