2010-11: Robby Fabbri skated for the Mississauga Rebels bantam AAA team that captured the Ontario Hockey Federation championship.
2011-12: Fabbri appeared in two junior A games with the OJHL's Toronto Junior Canadiens and was team captain of the Mississauga Rebels midget minor AAA team. He had 3 assists in a regular season game and was scoreless in an OJHL playoff contest with the Junior Canadiens. Fabbri scored 57 goals with 48 assists and had 75 penalty minutes in 60 games for the Rebels. Mississauga captured the championship of the 2012 OHL Cup and Fabbri scored 4 goals with 4 assists and 8 penalty minutes in seven games at the tournament. He was selected by Guelph in the first round (sixth overall) of the 2012 OHL Priority Draft.
2012-13: Fabbri skated for the OHL's Guelph Storm and represented Canada Ontario in the 2013 U17 World Hockey Challenge. He scored 10 goals with 23 assists and was +13 with 38 penalty minutes in 59 regular season games. The Storm lost to Kitchener in a first round series after finishing fourth in the Midwest Division. Fabbri had 1 assist and was -1 with 4 penalty minutes in five playoff games. He scored 3 goals with 1 assist and had 27 penalty minutes in five games for Canada Ontario at the WHC.
2013-14: Fabbri skated for Canada's gold medal U18 team at the Ivan Hlinka Memorial Tournament in August before returning to Guelph for his second OHL season. He had 1 assist and was -2 with 4 penalty minutes in five games at the Ivan Hlinka tournament. Fabbri was selected to play for Team Cherry in the 2014 CHL/NHL Top Prospects Game.