2010-11: Robert Hagg played for Tierps HK men's team in lower Swedish hockey as a 15-year-old, skating in 30 games for the East coast club, and played for MODO's U18 and U16 teams. He scored 2 goals with 9 assists and had 30 penalty minutes for Tierps HK. He was scoreless with 2 penalty minutes in two U18 games for MODO and scored 2 goals with 13 assists and 26 penalty minutes in eight games for MODO U16. Playing for Uppland in the TV-Pucken tournament for Swedish high school players Hagg led all defensemen with 4 goals with 5 assists in eight games.
2011-12: Hagg moved to MODO for the full season, spending most of the year with the U20 squad, and represented Sweden in the 2011 U19 World Junior A Challenge and 2012 U17 World Hockey Challenge tournaments in Canada. In 52 games for MODO U20, including eight playoff games, he scored 5 goals with 14 assists and was plus-11 with 48 penalty minutes. He played six games for MODO U18; scoring 1 goal with 4 assists and was minus-one. Hagg was scoreless with 4 penalty minutes at the WJAC and scored 2 goals with 4 assists and had 16 penalty minutes in six games at the WHC.
2012-13: Hagg split the season between MODO's Elitserien and U20 Super Elit league teams – playing in 27 games with the men's team and being named the top Super Elit defenseman. He played for Sweden in the 2013 U20 and U18 World Junior Championship tournaments, serving as an assistant captain on the U18 squad. Hagg had 1 assist and was minus-three, averaging 7:46 minutes of ice time, in Elitserien regular season play. Seventh place MODO played Farjestad in the playoffs but Hagg did not see any ice time in the five-game series. Hagg scored 11 goals with 13 assists in 28 games with MODO's U20 team during the regular season and played for both the MODO U20 and U18 teams in the playoffs (both of which won silver medals). He also won a silver medal with Sweden at the U20 WJC scoring 1 goal with 1 assist and was minus-one in five games. Sweden's U18 team reached the quarterfinals and in five games Hagg scored 1 goal with 3 assists and was plus-one with 12 penalty minutes. Hagg was ranked eighth amongst European skaters in Central Scouitng's final rankings prior to the 2013 NHL Draft.
Hagg is generally sound in most aspects of the game, but doesn’t yet seem to have any dominant traits nor has been able to be consistent in his effort. He keeps things simple, but can sometimes miss assignments in his own zone and won’t be found making breathtaking rushes with the puck on too many occasions. Hagg can play the point on the PP, but doesn’t stick out as an offensive dynamo or as a dangler of sorts. Sound first breakout pass. Has a bit of a choppy stride moving forward. Knees are usually bent while his shoulders are parallel to the ice and his back is extremely straight, all combining to give him a stiff appearance. A more quiet type who played at the U18, U20 and for MoDo of the SEL this past season. Has a great frame, is not shy of the rough stuff, and looks like he can be molded into a punishing defender, if a team is so inclined. Doesn’t back down from challenges and carries himself as if he’s unimpressed with the opposition.