After Harrington's senior season at Virginia, University of Minnesota Duluth (UMD) assistant head coach Mike Sertich urged Harrington to try out for the hockey team. With Sertich's backing, Harrington won a walk-on spot on UMD's roster. He lettered for four seasons at UMD from 1975-79.
Following his collegiate playing career, Harrington was invited to try out for the 1979-80 US National team coached by the Minnesota Golden Gophers' Herb Brooks. Harrington made every cut and was placed on the Olympic roster for the 1980 Lake Placid games. The team went on to defeat the highly favored Soviet Union 4-3 in the first game of the Medal Round in an event named the “Miracle on Ice.” The team defeated Finland 4-2 two days later to capture the Gold Medal for the US. Harrington is credited with an assist on Mike Eruzione's go-ahead game-winning goal against the USSR.
Undrafted by the NHL, Harrington had a try-out with the Buffalo Sabres following the Olympics. The Sabres immediately assigned him to the Rochester Americans of the AHL, where he scored seven points in twelve games despite being a marked man. "Some Canadian-born kids resent the fact that the Americans did so well in the Olympics and they're a bit jealous of them," claimed Buffalo defenseman Jim Playfair. In one game Harrington was struck in the face and needed six stitches to close a bloody gash on his chin. In another game against the Hershey Bears Harrington was attacked from behind by Lou Franceschetti and knocked unconscious for five minutes, laying in a pool of blood on the ice. He had to spend the night in hospital with a concussion, fractured jaw, broken nose and four loosened teeth. Franceshetti was not penalized, insisting it was a "clean" check. "My shoulder just connected with the back of his neck." "If it was a clean check, how did I get it in the back of my head?" asked Harrington. "Laying in my hospital bed, I had all sorts of mixed emotions, especially when my teeth ached and my heart throbbed. I kept thinking of the cleaner game in Europe, that I could have gone to Switzerland to play hockey. Something like this can't help but discourage you. But this is the way it is in the pros and I'll make it one way or the other ... even if I have to face a few goofballs."
Harrington played the 1980-81 season for Lugano in the Swiss league before returning to the US to play with the US National team from 1981-83 on a full time basis. He was a member of the United States team at the 1981, 1982 and 1983 Ice Hockey World Championship tournaments. Between brief stints in the Central Hockey League, Harrington rejoined the Olympic hockey team for the 1984 Winter Olympics.
From 1993-2008, Harrington was the head hockey coach at Saint John's University in Collegeville, Minnesota. Harrington turned around a listless program at Saint John's. By his fourth season he had guided SJU to a school-record 26-6-1 mark, the school's first conference championship since 1950, the school's first conference playoff championship, the school's first NCAA tournament berth and an NCAA third place victory. In 15 seasons at Saint John's he has a career record of 241-142-31 (.620). He has won five MIAC titles, four MIAC playoff championships and has advanced to the NCAA tournament five times. Harrington is the school leader in career wins. In addition to his coaching duties, Harrington was the assistant athletic director at Saint John's.
Prior to Miracle, Harrington was portrayed by Bill Schriner in the 1981 made-for-tv movie Miracle on Ice.