Dumba, a native of Regina, Saskatchewan, led all WHL defensemen in goals during the 2011-12 season with 20, and ranked sixth in points with 57 in 69 games. He was ranked No. 11 among North America skaters by NHL Central Scouting and No. 6 overall by TSN. He won Rookie of the Year in the WHL in 2010-11 after leading the Rebels in rookie scoring, and finishing seventh overall among first year players with 15 tallies. Dumba was also invited to Canada's 2012 World Junior Championship team camp. Though he was not selected to play, he was the youngest invited.
He was captain of Canada's 2012 Under-18 team at the U-18 World Championships, where he scored 12 points in seven games en route to a bronze medal.