As a junior curler, Sundgren played for two curling teams before he entered university. At age 16, he played in Sveg, curling in the third position for a team skipped by Johan Eriksson. The following year, he joined the team based in Lit and led by brothers Kristian Lindström and Alexander Lindström and rounded out by Henrik Leek. In 2008, the team invited Oskar Eriksson to join, who eventually took over the role of skip while playing in the third position. With the newly formed team, Sundgren played lead and competed at the 2009 World Junior Curling Championships, finishing in third place in the standings after the preliminary rounds but in fourth place overall, losing to the US team in the semifinal and the bronze medal game, though they had defeated the US team earlier in the tournament. The team had immediate success on the junior tour and also scored wins in the senior division.