Jimmi Simpson (born November 21, 1975) is an American actor. He is best known for his work on television, which includes recurring roles as Liam McPoyle on It's Always Sunny in Philadelphia (2005–2013), Lyle the Intern on The Late Show with David Letterman (2008–09), Mary Lightly on Psych (2009–13), Lloyd Lowery on Breakout Kings (2011–13), Jack Spaniel on The Newsroom (2014), Gavin Orsay on House of Cards (2014–15), and the younger William on Westworld (2016–2020). For the last role, he received a nomination for the 2018 Primetime Emmy Award for Outstanding Guest Actor in a Drama Series. In 2022, he starred in a television series adaptation of The Man Who Fell to Earth.

Simpson also starred as Soldier on SundanceTV's Hap and Leonard (2016), Detective Russell Poole on USA's Unsolved (2018), and James Schaeler on ePix's Perpetual Grace, LTD (2019). He received a nomination for the 2018 British Academy Television Award for Best Supporting Actor for his performance in the Black Mirror episode "USS Callister" (2017).

Simpson made his feature film debut in Loser (2000). Subsequent film roles have included Herbie: Fully Loaded (2005), Stay Alive (2006), Seraphim Falls, Zodiac (both 2007), The Invention of Lying (2009), Date Night (2010), Abraham Lincoln: Vampire Hunter (2012), White House Down, Knights of Badassdom (both 2013), Under the Silver Lake (2018), Unhinged (2020), Breaking News in Yuba County, and Silk Road (both 2021). On stage, Simpson's portrayal of Philo Farnsworth in The Farnsworth Invention (Broadway, 2007–08) earned him a Theatre World Award.

Early life
Simpson was born in Hackettstown, New Jersey, on November 21, 1975. He has two older brothers. He attended Hackettstown High School, where he took his first acting class. After graduating from Bloomsburg University with a BA in theater, he acted for four seasons at the Williamstown Theatre Festival in Williamstown, Massachusetts.