Schenn began his hockey career with the Kelowna Rockets of the Western Hockey League in 2005–06. He was Kelowna's first pick in the 2004 WHL Bantam Draft. Schenn made his WHL debut on September 23, 2005 against the Vancouver Giants. Later in his first season, on October 11, 2005, he notched his first WHL goal in his hometown against the Saskatoon Blades. He was an alternate captain for the Rockets for two seasons.
In his brief junior career, Schenn has had the opportunity to play for Canada on several occasions. He was a member of Team Canada at the 2007 Super Series, where he helped Canada go undefeated for eight games against Russia. He also played for Team Canada at Under 18 World Cup Tournament, where Canada won gold. During the 2007–08 season, Schenn was named to the WHL's roster for its part in the ADT Canada-Russia Challenge.
Also during the 2007–08 season, Schenn was named to Team Canada's selection camp roster for the 2008 World Junior Hockey Championships. Schenn was named to the team on December 13, 2007. Canada won the gold medal and Schenn finished the tournament with a team high +/- rating with a +5.
Schenn was selected in the first round, fifth overall, by the Toronto Maple Leafs in the 2008 NHL Entry Draft. On October 7, 2008, Schenn signed his first professional contract with the Maple Leafs, after impressing the Leafs' coaching staff in the 2008 preseason. Schenn has drawn early comparisons to Maple Leafs legendary defenceman Tim Horton. On Monday, October 27, 2008, Schenn was told by the Leafs coach Ron Wilson that he would remain with the Toronto Maple Leafs for the entirety of the 2008-09 NHL season. By playing more than nine games in his first season, Schenn's contract will begin to count to restricted free agent status at the end of the contract. Schenn acquired his first NHL point in a game against the New Jersey Devils in a 6 - 5 shootout win. Schenn suffered the first injury of his young NHL career in a December 6, 2008 game against the Washington Capitals. Schenn suffered a lower-body injury that will keep him out of the active roster for at least two weeks.