2009-10: Fabrice Herzog was an assistant captain for the Pikes Oberthurgau U17 in Switzerland's Elite Novizen as a 14-year-old after appearing in 17 games with the team the previous season. He scored 6 goals with 7 assists in 32 games and had 12 penalty minutes. Herzog scored 6 goals in six post-season games as Pikes Oberthurgau avoided relegation.
2010-11:Herzog appeared in three lower league men's games with Pike Oberthurgau's first and second teams and scored 22 goals in 27 games for the club's U17 team. He had 1 assist and 2 penalty minutes in three men's games. Herzog was the third-leading scorer for the U17 team, adding 14 assists to his 22 goals. He led the team in scoring in the relegation playoffs, scoring 6 goals with 3 assists in six games.
2011-12: Herzog skated for Zug in Switzerland's Elite National junior league and represented Switzerland in the 2012 U18 World Junior Championship. He scored 18 goals with 14 assists in 35 games with Zug's junior team and had 45 penalty minutes. Zug finished second and reached the playoff finals against Swiss champion GCK/ZSC Lions. Herzog scored 6 goals with 2 assists in ten playoff games. In six games for Switzerland's U18 team at the WJC he scored 2 goals and was -2.
2012-13: Herzog played 20 games in Switzerland's National A League for Zug and was the third-leading goal scorer in the Swiss Elite Jr A league while playing for Zug's U20 team. He scored 2 goals with 2 assists and was +5 with 6 penalty minutes in 20 games with the Zug men's team. Herzog scored 28 goals with 17 assists and 26 penalty minutes in 32 regular season junior games. In four playoff games he scored 3 goals with 2 assists. Ranked 59th amongst European skaters in Central Scouting's final rankings, Herzog was selected by Toronto in the fifth round (142nd overall) of the 2013 NHL Draft.