Mummery played professionally from 1911 until 1923, including six seasons in the National Hockey League (NHL) for the Toronto Arenas, Quebec Bulldogs, Montreal Canadiens and Hamilton Tigers.

Mummery's six assists as an NHL rookie in the 1918 Stanley Cup Finals set an NHL-era record still unbroken.

Unfortunately, the Arenas only managed to win five of their games throughout the 18-game schedule, and with poor attendance figures the team officially withdrew from the league on February 20, 1919.

According to the Canadian sports journalist Elmer Ferguson, who first saw Mummery play at the Westmount Arena in Montreal, the big defenceman had a prodigious appetite for food and a particular fondness of large steaks and cream.

Ferguson claimed that Mummery had testified to drinking a quart of cream after every meal because he figured that it gave him strength.

During the 1920–21 NHL season he scored 15 goals in 24 games for the Montreal Canadiens, many of them from what Elmer Ferguson referred to as a "one-man-power-play.