After taking an accelerated course load to finish high school early, Mersch joined the Wisconsin Badgers at the University of Wisconsin–Madison in 2010.

Mersch finished his collegiate career by signing an entry-level contract with the Los Angeles Kings in April 2014.

[2] Before his father died in 2000 from cancer,[2] Mersch and his younger brother Dominick were motivated by him to play ice hockey.

After his successful rookie season with the USNTDP, Mersch garnered the attention of Mike Eaves, the head coach for the Wisconsin Badgers men's ice hockey team.

[4] Mersch scored his first collegiate goal on October 10, 2010, to help the Badgers shut-out the Holy Cross Crusaders 6–0.

[16][17] Mersch's efforts were recognized with the Western Collegiate Hockey Association's (WCHA) Offensive Player of the Week after he recorded three goals and six points over two games at the start of March.

[21] Once the season was over, Mersch attended the Los Angeles Kings 2013 Development Camp,[22] but intended to return to UW–Madison for his senior year.

[21] In his final year with the Badgers, Mersch led the team in goals, set a program record, and earned numerous Big Ten accolades.

[23] After scoring again the following game to maintain a five-game goal streak, Mersch earned a Big Ten Second Star of the Week honor on December 17.

[29] On April 1, 2014, Mersch concluded his collegiate career by signing a three-year entry-level contract with the Los Angeles Kings.

[30] After finishing his college career, Mersch was assigned to the Kings' American Hockey League (AHL) affiliate, the Manchester Monarchs, for the remainder of the 2013–14 season.

[35] Mersch and the Monarchs finished the 2014–15 season with a league-best 50–17–6–3 record and met with the Portland Pirates in the first round of the 2015 Calder Cup playoffs.

[39] After eliminating the Penguins, Mersch ranked ninth on the Monarchs’ all-time playoff points leaders with six goals and eight assists.

[40] Over the first two games of the Eastern Conference Finals, Mersch scored five goals to lead the Monarchs to two consecutive wins over the Hartford Wolf Pack.

[41] After eliminating the Wolf Pack in four games, Mersch and O’Neill tied for fourth place on the team’s all-time playoff points list with 10 goals and nine assists.

[44] Mersch again participated in the Los Angeles Kings development camp but reassigned to the Ontario Reign for their inaugural 2015–16 season.

[51] Mersch spent a month with the Reign before being recalled to the NHL again on February 19, as a replacement for an injured Anže Kopitar.

[52] The call-up only lasted two games as the Kings acquired veteran Kris Versteeg in a trade and Mersch was subsequently returned to the Reign.

His efforts also helped the Reign qualify for the 2016 Calder Cup playoffs but he struggled in their first-round matchup against the San Jose Barracuda.

He signed a one-year, two-way contract extension with the Kings on July 14, 2017, worth an Average Annual Value of $650,000 at the NHL level.

[59] Following the signing of the contract, Mersch again participated in the Los Angeles Kings' development camp before being reassigned to the Ontario Reign for the 2017–18 season.

[60] Upon returning to the Reign, Mersch was named an alternate captain alongside Justin Auger, Andrew Crescenzi, and Kevin Gravel.

[62] After four full seasons within the Kings organization and having been unable to break into the NHL squad, Mersch left as a free agent and signed a two-year, two-way contract with the Dallas Stars on July 1, 2018.

[70] During Game 3 of the 2023 Calder Cup Eastern Conference series, Mersch suffered a head injury following a check by Utica Comets player Robbie Russo.