Denneny played senior hockey in Cornwall, starting with the Cornwall Sons of England of the Lower Ottawa Valley hockey league in 1909–10. 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). He had tried out for the
Montreal Canadiens in 1912 but failed to make the team and he returned to senior hockey. He was traded to the Ottawa Senators in 1916 and he would play with the Senators until 1928. . With the Senators during the 1917–18 season, Denneny set an NHL record by opening the season with four straight multi-goal games, a record that was tied in
2013 by
San Jose Sharks' forward
Patrick Marleau. Denneny was a member of four Senators Stanley Cup-winning teams; in 1920, 1921, 1923 and 1927. He faced his brother Corbett during the 1923 Stanley Cup playoffs, a series which also featured brothers
Frank and
Georges Boucher. This marked the first time two different sets of brothers faced each other in an NHL or
Big Four championship series. Denneny was sold to Boston in 1928, where he would be an assistant playing-coach of the Bruins' 1929 Stanley Cup-winner. In 1929, Denneny retired to become an NHL on-ice official. 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. The team finished last and Denneny was not retained as coach. Denneny was one of the top scorers in the NHL from 1917 through 1925. While leading the league in scoring during the
1923–24 NHL season, he did so by recording 22 goals and one assist for a total of 23 points, the lowest winning total in NHL history. When he retired, he was the all-time top scorer in NHL history. He was inducted into the
Hockey Hall of Fame in 1959. In 1998, he was ranked number 62 on
The Hockey News'
list of the 100 Greatest Hockey Players. He was the first and fastest player in NHL history to score 200 goals (181 GP). During a six-week span in the
1920–21 NHL season, Cy and his brother
Corbett (
Toronto St. Patricks), each scored six goals during a game—a feat accomplished by only
five other players in the history of the NHL. ==Playing style==