Undrafted during his four years at Bowdoin College in Boston, Ahearn was an aspiring teacher and hockey coach before he was signed by the California Golden Seals as a free agent in September 1974.
He played most of 1974-76 with Salt Lake in the CHL, making a brief appearance the first time around with the Seals and staying for half the season the second time he was called up. He played for Team USA at the 1976 Canada Cup, and then stayed with the Seals as they moved to Cleveland.
A year and a half later, on January 9, 1978, he was traded to another basement team, the Colorado Rockies, but after half a season he was out of the NHL. In five seasons and 146 games, he played only twice in the playoffs before the Rockies were quickly eliminated from the 1978 post-season.
Ahern played for two more years in the minor pro leagues of smalltown USA and then spent a year in Switzerland before returning to Cape Cod for one last season in the ACHL.