Harrington was drafted by the Pittsburgh Penguins in the second round of the 2011 NHL Entry Draft and was traded to the Toronto Maple Leafs as a part of the package that landed the Penguins Phil Kessel. Injuries kept Harrington out of the lineup as he was only able to play in 32 games across both the NHL and AHL levels.
The Maple Leafs then traded him to the Blue Jackets for Kerby Rychel and that's where he ended up spending the bulk of his career. From 2017-2022 he appeared in 178 NHL games where he scored 7 goals and 36 points.
He ultimately walked away from Columbus as a free agent and agreed to a professional tryout with the San Jose Sharks. He earned himself a contract but has only played in 2 game this season, recording an assist in 15:48 ATOI.