Selected by the Pittsburgh Penguins in the second round (No. 54) of the 2011 NHL Draft, Harrington played two seasons with Wilkes-Barre/Scranton of the American Hockey League and made his debut with the Penguins on Dec. 18, 2014.
Harrington was traded to the Toronto Maple Leafs on July 1, 2015, in a deal that sent Phil Kessel to the Penguins. Harrington had one assist in 15 games with Toronto and was then traded to the Blue Jackets on June 25, 2016. He scored three points in 32 games with Columbus, including his first NHL goal against the Ottawa Senators on Jan. 22, 2017.