2008-09: Mersch joined the US National Development Team Program's U-17 squad after competing for Team Illinois the previous year. In 65 games he was the team's fourth-leading scorer with 27 goals with 19 assists and had 58 PMs. Mersch committed to attending the University of Wisconsin in 2010-11.
2009-10: Mersch skated in 49 games with the US National Development Team U18 program while accelerating his studies at Ann Arbor's Pioneer High School. He struggled offensively, scoring 4 goals with 10 assists and 30 PMs while developing a more well-rounded game. Mersch graduated high school in three years to be able to play college hockey rather in 2010-11 than spending a season in the USHL.
2010-11: Mersch forced his way into full-time role with Wisconsin as an 18-year-old freshman; skating in 41 games and providing both timely offense and determined physical play. Mersch scored 8 goals with 11 assists and was +23 with 32 PMs as the Badgers finished seventh in the WCHA during a roller coaster season. The Badgers won 12 of 13 games during one mid-season stretch (including splitting a series with eventual national champion Minnesota-Duluth) but then won just two of their last 11 games.
2011-12: Mersch was the third-leading scorer for Wisconsin as a sophomore. Skating in all situations for the Badgers, he scored 14 goals with 16 assists and was minus-five with 37 penalty minutes in 37 games. Seven of Mersch's goals were scored on the power play. Wisconsin struggled to break the .500 mark, finishing an uncharacteristic 10th in the WCHA. The Badgers forced Denver to overtime in the decisive game of their three-game, first-round series in the WCHA tournament before falling 3-2. Mersch struggled against the powerful Pioneers; finishing minus-four with no points.