His professional playing career began with the Toronto Ontarios/Shamrocks of the National Hockey Association (NHA) in 1914 (The name of the team changed during the season).

This marked the first time two different sets of brothers faced each other in an NHL or Big Four championship series.

In 1932, he re-joined the Senators as head coach, but the team was in decline due to financial difficulties which forced management to sell top players in order to survive.

Despite not being a swift skater, Denneny had one of the most deceptive and accurate shots in the league, which enabled him to achieve his scoring feats so rapidly.

[4] He was one of the first known players to use opposing defencemen as screens,[5] and would beat goaltenders with head fakes and subsequently with shots that often would not leave the ice.

[4][7] He was a very physical player who often acted as an enforcer for his linemates, Jack Darragh and Frank Nighbor.