He was a champion rower for Bishop Timon High School in Buffalo, and the West Side Rowing Club.

He began swimming competitively as a 17-year-old freshman at Cortland State University under Hall of Fame Coach Doc Counsilman, almost a decade later than many of his future rivals.

[1] Many consider Breen's most significant single swim time, and greatest single effort, his 1500-meter Long Course World Record of 18:05.9 at the May 3, 1956 U.S. AAU Indoor Championship at Yale, referred to by Ohio State Coach Mike Peppe as “the single most brilliant effort in swimming since I’ve been coaching.” Breen reduced the standing world record by a substantial 13.1 seconds, and incredibly completed his swim a full 1:18, ahead of the second-place finisher, Frank Brunell, who had been an American National Champion many times.

As a member of the second-place U.S. team in the men's 4×200-meter freestyle relay, Breen earned a silver medal, together with Dick Hanley, Bill Woolsey and Ford Konno.

[1] Breen was a resident of Washington Township, Gloucester County, New Jersey, where he coached local high school and club swim teams into his 70s.