Ellis finished his junior career as one of the most decorated players in Canadian Hockey League history.

He won the Memorial Cup with Windsor of the Ontario Hockey League (one of three minor leagues that make up the CHL) in 2009 and 2010. Ellis played for Canada in three straight IIHF World Junior Championships, winning a gold medal in 2009 and a silver medal in 2010 and 2011. He also won gold with Canada at both the 2008 Under-18 World Championship and the 2008 Ivan Hlinka Memorial Cup, and was a tournament all-star and won gold at the 2008 World Under-17 Hockey Challenge with Team Ontario.

Ellis had 314 points in juniors, becoming the third defenseman (Rick Corriveau, Denis Potvin) to reach 300 points in the OHL. He won the 2011 Red Tilson Trophy as the league's outstanding player and was the CHL player and defenseman of the Year in 2010-11. He won the Max Kaminsky Trophy as the OHL's top defenseman in 2008-09 and 2010-11. He also won the 2011 Mickey Renaud Captain's Trophy for leadership on and off the ice. With 101 points in 2010-11, Ellis became the first OHL defenseman in 17 years to score 100 points.

Selected by the Nashville Predators at No. 11 in the 2009 NHL Draft, Ellis began his pro career in 2011-12 with Milwaukee of the American Hockey League but was called up to Nashville on Dec. 26, 2011, making his NHL debut that night against the Detroit Red Wings. He scored his first NHL goal Jan. 7, 2012, against the Carolina Hurricanes. The Predators went 22-8-2 with Ellis in the lineup. He made his Stanley Cup Playoff debut April 15, 2012, against the Detroit Red Wings at Joe Louis Arena.

Ellis split the 2012-13 season between Nashville (32 games) and Milwaukee (32 games). He suited up for the Predators' first 31 games and then was assigned to Milwaukee on March 22, 2013. He returned to play one more game for Nashville on April 4, 2013.

In 2013-14, Ellis ranked second on the Predators in games (80) and plus/minus rating (plus-9) and was third among Nashville defensemen with 27 points. He played in his first IIHF World Championship, leading Canada defensemen with five points.

His nine goals and 27 points in 2014-15 included his first three-point game in the NHL, on Oct. 21, 2014, against the Arizona Coyotes. Ellis scored his first shorthanded NHL goal March 29, 2016, against the Dallas Stars.

Ellis scored an NHL career-high 16 goals in 2016-17, then had 13 points (five goals, eight assists) in 22 games as the Predators advanced to the Stanley Cup Final for the first time since entering the NHL in 1998.

The following season Ellis scored 32 points (nine goals, 23 assists) in 44 games, and on Aug. 14, 2018, he was rewarded with an eight-year, $50 million extension ($6.25 million annual average value).

Ellis continued to be a key defenseman for Nashville the following two seasons, but after scoring 18 points (five goals, 13 assists) in 35 games in 2020-21, he was acquired by the Philadelphia Flyers in a three-team trade involving the Predators and Golden Knights on July 17, 2021.

Ellis was traded to the Chicago Blackhawks by the Sharks on Jan. 8, 2026. He had been sent to San Jose by Philadelphia on Oct. 5, 2025.