After his second season in the Ontario Hockey League (OHL), Ellis was selected 11th overall by the Predators in the 2009 NHL Entry Draft.

During his junior career, he was key in guiding the Windsor Spitfires to back-to-back Memorial Cups as Canadian Hockey League (CHL) champions in 2009 and 2010.

He was named to the Canadian Hockey League (CHL) and OHL All-Rookie Teams,[4] along with Spitfires teammate Taylor Hall.

[7] Ellis was further recognized for his academic performance, maintaining an 81 percent average in university preparation courses at St. Anne Catholic High School in Belle River, earning him the League's Bobby Smith Trophy as scholastic player of the year.

[3][14] Improving to 22 goals, 67 assists and 89 points in 57 games in 2008–09, Ellis led all OHL defencemen and finished second in team scoring to Taylor Hall.

[3][17] As a result, Ellis was named to the OHL First All-Star Team and awarded the Max Kaminsky Trophy, beating fellow nominees P. K. Subban and Cameron Gaunce as the League's top defenceman.

[3][4] The distinction made him a finalist for CHL Defenceman of the Year, but he lost to Jonathon Blum of the Western Hockey League (WHL)'s Vancouver Giants.

[18] Ellis was additionally voted the smartest player, hardest shot and best offensive defenceman in the Western Conference in an annual poll of OHL coaches.

Ellis added 31 points in 20 games, tying for third in post-season scoring[20] while helping the Spitfires to a J. Ross Robertson Cup as OHL champions.

Earning a berth in the 2009 Memorial Cup, the Spitfires went on to win the national major junior championship over the Kelowna Rockets.

[14] Addressing this supposed weakness, Hall of Fame defenceman Bobby Orr said of him, "he's not very big, but you'll see...he's a heck of a hockey player.

[19] Ellis subsequently attended his first NHL training camp with Nashville in September 2009, but was returned to Windsor after suffering a wrist injury.

[24] Finishing the season with 61 points over 48 games in 2009–10, he ranked fourth among League defencemen in point-scoring[25] and was named to the OHL Second All-Star Team.

Ellis beat out Simon Després and Stefan Elliott for the former recognition, while being voted over Sean Couturier and Darcy Kuemper for the latter.

[33] Upon joining the Admirals, he immediately made an impact by tallying his first assist 10 minutes and 30 seconds into Game 1 of the West Division Finals.

[43] He played 13:37 minutes of ice time in his debut that night and was minus-one with two shots on goal against the Detroit Red Wings.

As he began earning the trust of head coach Barry Trotz, Ellis also saw time on the second power-play unit with Roman Josi.

[50] As the Predators qualified for the 2012 Stanley Cup playoffs, Ellis made his postseason debut in Game 3 of the Western Conference Quarterfinals against the Detroit Red Wings.

[54] Once the lockout ended, Ellis participated in the Predators training camp[55] and was named to their opening night roster for the shortened 2012–13 season.

[60] Prior to the start of the 2013–14 season, the Predators hired Phil Housley as their assistant coach in part to help Ellis and the other young defencemen improve their game.

His 42-game scoring drought came to an end on January 15 after he tallied a goal and an assist in the third period to lead the Predators to a 4–2 win over the Calgary Flames.

Although he often played as a bottom defenceman, Ellis averaged 14:27 per game in time on ice while also manning the second power-play unit with rookie Seth Jones.

[81] With nine points in eight games, Ellis was the tournament's leading defenceman[82] and was named to the All-Star Team, along with Ontario teammate Matt Duchene.

[85] Four months later, he re-joined the national under-18 team for the 2008 Ivan Hlinka Memorial Tournament, held in Slovakia and the Czech Republic.

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.

[94] As a two-time veteran of Canada's junior team, Ellis opted not to attend their annual summer evaluation camp.

[97] 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.

[99] Playing Russia in the gold medal game, Ellis scored the opening goal in a 5–3 losing effort,[100] earning silver for the second consecutive year.

Known as a power play specialist, he often "quarterbacks" from the blueline—controlling puck distribution between teammates in the offensive zone—and possesses a hard and accurate shot.

[8] In February 2010, Ellis was selected as his hometown Hamilton/Burlington area's 2009 Golden Horseshoe Athlete of the Year, beating out college quarterback Danny Brannagan and marathon runner Reid Coolsaet.