Kaitlin Willow McElhenney (née Olson; born August 18, 1975) is an American actress and comedian. She began her career in the Groundlings, an improvisational group in Los Angeles, California, and had minor roles in several television series before being cast as Deandra "Sweet Dee" Reynolds on the FX black comedy series It's Always Sunny in Philadelphia in 2005.
She has also appeared in film, with roles in Leap Year (2010), The Heat (2013), and also provided the voice for Destiny in Finding Dory (2016). In 2017, she began playing the lead role of Mackenzie "Mickey" Molng on the Fox comedy series The Mick.
Olson was born in Portland, Oregon to Donald Lee Olson and Melinda Leora. She resided on Vashon Island, Washington, in Puget Sound, 13 mi (21 km) west of Seattle, until the age of eight. Her family relocated back to the Portland area, settling in Tualatin, where Olson grew up on a farm. Her father, Donald Olson, worked as the publisher of the Portland Tribune from 2000 to 2001. At age twelve, Olson was involved in a serious bicycle accident involving a vehicle, resulting in a fractured skull requiring reconstructive surgery. She graduated from Tigard High School in Tigard, Oregon in 1993.