John Gallagher Jr. has appeared in several television programs, including The West Wing (1999), Law & Order: Special Victims Unit (1999), NYPD Blue (1993), Ed (2000), Love Monkey (2006), and Hallmark Hall of Fame's film The Flamingo Rising (2001). He played Jim Harper in Aaron Sorkin's The Newsroom (2012) which aired on HBO. He also appeared in HBO's mini-series Olive Kitteridge (2014), based on the Pulitzer-winning novel of the same name, in the role of Christopher Kitteridge.
Film credits include Pieces of April (2003), Woody Allen's Whatever Works (2009), Jonah Hex (2010), The Heart Machine (2014), Kenneth Lonergan's Margaret (2011), and a lead role in Short Term 12 (2013) opposite Brie Larson. In 2016, he starred in the thriller film 10 Cloverfield Lane (2016) and the horror thriller film Hush (2005).