Selected by the Buffalo Sabres in the first round (No. 26) of the 2008 NHL Draft, Ennis got started quickly in the NHL by scoring 20 goals in 2010-11, his first full League season.
Ennis had shown his scoring touch early on with consecutive 43-goal seasons for Medicine Hat of the Western Hockey League in 2007-08 and 2008-09. He led Medicine Hat in all major scoring categories in 2007-08, and skated in the Canadian Hockey League Top Prospects Game. Ennis served as an alternate captain for Team WHL during the 2008 Canada-Russia Challenge and was named the WHL's Most Sportsmanlike Player and selected to the WHL East Division First All-Star Team.