2010-11: Alexander Wennberg made the jump from Boo IF in Sweden's Division 2 to the Djurgarden club in elite juniors. In 40 games for the Djurgarden U18 team he scored 11 goals with 23 assists. In five playoff games he had 1 goal with 2 assists and 2 penalty minutes as Djurgarden advanced to the U18 finals.
2011-12: Wennberg moved up to Djurgardens' U20 team and represented Sweden in the 2011 U19 World Junior A Challenge and 2012 U18 World Junior Championship tournaments. He scored 1 goal with 19 assists in 45 games, including three playoff contests, and was plus-six with 6 penalty minutes. Wennberg played eight games for Djurgardens' U18 team. In four games for Sweden at the WJAC he was scoreless with no penalty minutes. Wennberg had a breakout tournament for Sweden's silver medal-winning U18 team, scoring 3 goals with 6 assists and finishing plus-two in six games. He spent time with Djurgarden's Elitserien team but did not appear in any games.
2012-13: Wennberg spent his first season in Swedish pro hockey as an 18-year-old, skating in 46 games for Djurgarden, which had been demoted from the Elitserien to Allsvenskan. Including Colorado's Gabriel Landeskog, who spent time with the club during the NHL lockout, Djurgarden featured several teenagers. Wennberg was the team's fourth-leading scorer, scoring 14 goals with 18 assists in 46 games, and was plus-10 with 14 penalty minutes. In three Kval playoff games he had 3 assists. Playing for Sweden's silver medal U20 team at the 2013 World Junior Championship he scored 2 goals with 1 assist and was plus-two in six games. In April he signed a two-year contract to play for Frolunda in the Elitserien. Wennberg was ranked fifth amongst European skaters in Central Scouting's final rankings prior to the 2013 NHL Draft.