Dennis grew up in Etobicoke where he started playing hockey at eight years of age. He began his OHA career with the Toronto Marlies and was later traded to the London Knights.

In the 1975-76 season he was drafted by the California Golden Seals, which after its demise, resurfaced as the Cleveland Barons. That club folded and was later absorbed by the Minnesota North Stars, Dennis included.

 

Dennis was later traded to the Washington Capitols and became one of the NHL's top scorers. In his second full season with the club he scored 50 goals. The next year, 1981, he scored 60 goals and 76 assists for a total of 136 points. Today only eight players in history have equalled or surpassed his single season point totals.

Dennis finished his NHL career with the Minnesota North Stars in 1989, ending a stellar 16-year career.

Dennis has three children and lives with his wife, Kimberly, in Aspen, Colorado, where he works at the historic Hotel Jerome.