Harrington was born and raised in Wellesley, Massachusetts to Judy and Terry Harrington. He has two brothers, actor Adam and Matt. As a child, his family spent summers on Cape Cod up the road from The Harwich Junior Theater. He attended Wellesley High School and then studied theater at Syracuse University.
Harrington has been featured as Dr. Simon O'Keefe on the WB drama Summerland, FBI Special Agent Paul Ryan on FOX's short-lived The Inside, and Steve on NBC's version of Coupling. He appeared in a recurring role on the police drama series The Division. He has also had roles on A.U.S.A., The Shield, Time of Your Life, Private Practice, and Burn Notice.
In 2006, Harrington began appearing as Dr. Ron McCready on the series Desperate Housewives. He starred as Ted Crisp in the comedy series Better Off Ted from March 2009 to January 2010. He had a recurring role in Season 4 of the TV Land comedy series Hot in Cleveland. In 2014, he co-starred on the short-lived USA Network comedy series Benched. In 2017, he had a guest role on Suits as Mark Meadows, Donna's ex-boyfriend. He currently stars as Sergeant II David "Deacon" Kay on S.W.A.T. based on the 1975 television series and 2003 movie of the same name.

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