Described as the Wayne Gretzky of his era,[2] Cowley twice won the Hart Memorial Trophy as the NHL's MVP, and is widely regarded as one of the best playmakers in hockey history.

[8] After playing a single minor professional game for the Tulsa Oilers of the American Hockey Association at the start of the 1934–35 season—scoring no points but getting into a fight—Cowley broke in as a rookie with the St. Louis Eagles, formerly the Ottawa Senators.

[11] By the playoffs, he had improved enough to be a starter, centering a checking line with Paul Runge and Peggy O'Neil, and contributed two goals in the Bruins' two-game total-goals loss to the Toronto Maple Leafs.

[14] While the injury-riddled Bruins failed in the playoffs again—losing in three games to the Montreal Maroons—Cowley was rewarded by a fourth place finish in voting for the Lady Byng Memorial Trophy.

Again Cowley led Boston in scoring with 38 points, good for fifth in the league, and he was named a First Team All-Star at center for the first time.

While the Bruins were comfortably in first place through the season's halfway point,[23] all three of the Kraut Line were called up to the Canadian armed forces shortly thereafter.

[27] He was named First Team All-Star for the third time, and given the war- and injury-riddled Bruins' roster, was awarded his second Hart Memorial Trophy as the league's most valuable player.

The injury bug struck again on January 7, when in the midst of a 12–3 rout by Toronto, Leaf center Jack McLean separated Cowley's shoulder with a heavy boardcheck; Bruins manager Ross alleged that it was a dirty play and the result of a deliberate attack.

[31] Upon returning to the lineup, he reinjured his chronically bad knee and was forced out again, eventually missing 14 games in all to finish seventh in league scoring.

[32] Despite missing so much action, he was named First Team All-Star at center for the fourth and final time, and was runner-up in Hart Trophy voting to Babe Pratt of Toronto.

[35] Centering Cain and Don Gallinger as the 1945–46 NHL season dawned,[36] and with the return of many players from the military, Cowley started out strong, scoring 24 points in 26 games before suffering a compound fracture of the wrist in a January match against the Canadiens.

In his final season, Cowley was slowed by age and the progressive effect of his many injuries, but finished fourth in team scoring behind Schmidt, Bauer and Dumart despite missing nine games, and centered a line with Gallinger and Bep Guidolin.