Moving on to the Canada's under-20 team, he made his first of three World Junior Championship appearances in 2009. Named to the team at 17 years old, he was one of three draft-eligible players on the club with John Tavares and Evander Kane. Head Coach Pat Quinn had coached Ellis previously at the World U18 Championships. He selected Ellis to the team as the eighth defencemen, instead of choosing a traditional 13th forward, because of his abilities on the power play. In the semifinal against Russia, Ellis was instrumental in tying the game with five seconds remaining in regulation. Twice, he preventing the puck from being cleared out of the offensive zone, which allowed a play to develop that resulted in Jordan Eberle's game-tying goal. Canada beat Russia in a shootout before defeating Sweden in the final to win gold. Ellis scored seven points over six games to finish fourth among tournament defencemen in scoring. It also set a Canadian record for most points by a 17-year-old defenceman.