Ahern grew up playing in the South Boston Youth Hockey League before attending Boston Technical High School. He then went undrafted by the NHL after four years at little known Bowdoin College. This was in 1974, when it was pretty rare to see American players in the NHL, let along college players, especially from schools not known for hockey like Bowdoin's Polar Bears team. In fact only Weymouth, Massachusetts' Bobby Sheehan was the only New Englander in the NHL at the time.
Ahern probably was happy enough just playing hockey in school while aspiring to become a teacher. Maybe he would stay in the game as a youth hockey coach. That all changed when he was invited to the 1974 NHL training camp of the California Golden Seals. He would sign with the Seals, playing most of the next two seasons with their farm team in Salt Lake City, but also appearing in 47 NHL contests, even scoring a respectable 19 goals and 28 points.
His first NHL goal was a highlight of his career, scoring against Bernie Parent of the Flyers. But an equal highlight was scoring the game winning goal against Gerry Cheevers in his first visit into Boston.
Teammates took a while to warm up to him, nicknaming him "Preppy" as he had GQ good looks and the college education. But they quickly found out he was a tough son-of-a-gun and was not afraid to mix it up on the ice.
When the Seals relocated to Cleveland to become the Barons, Ahern went along with them, too. He must have missed the California sunshine as his offense did not follow him. He scored just 7 goals and 15 points in 61 games with the Barons, though he did score the first goal in Cleveland's NHL history.
He was traded to the Colorado Rockies in January, 1978. He played 38 games in Denver, scoring 5 goals and 18 points, before disappearing to the minor leagues forever in 1978. A badly broken arm may have scared NHL teams away.
Ahern was also chosen to represent the United States at the inaugural Canada Cup tournament in 1976. He even scored twice in five games.
Following his hockey career Ahern returned to Boston with his wife, a one-time professional model. He would get a job in the mayor's office. Though he never played for the Bruins he is active with their alumni team.