As a two-time veteran of Canada's junior team, Ellis opted not to attend their annual summer evaluation camp. In December 2010, he was named to the squad as one of four returning players for the 2011 World Junior Championships, held in Buffalo, New York. Ellis became the seventh player in Canadian history to play three times in the tournament. He was later chosen as team captain. Ellis began the tournament tied with Pietrangelo as the country's all-time top-scoring defenceman at the World Juniors. By scoring a goal in the opening game against Russia, a 6–3 win for Canada, he broke the tie to take the all-time scoring lead among Canadian defencemen. Later in the tournament, Ellis recorded three assists in a game against Norway, making him the World Juniors' all-time leading scorer among defencemen, passing Finnish defenceman Reijo Ruotsalainen's mark of 21 points. In the quarterfinal, he assisted on a goal against Switzerland to break Eric Lindros's 19-assist record as Canada's all-time leader at the tournament. Playing Russia in the gold medal game, Ellis scored the opening goal in a 5–3 losing effort, earning silver for the second consecutive year. Finishing the tournament with three goals and ten points to lead all blueliners, he earned Best Defenceman and All-Star Team honours. He was additionally voted by tournament coaches as one of Canada's best three players. With 25 points over three years at the tournament, he tied Jordan Eberle as Canada's second-best scorer of all-time, behind Lindros' 31 points.