Following the conclusion of his major junior career with the Memorial Cup winning Generals after the 2014–15 season, Mermis' unconventional development path was rewarded as he was signed as a free agent to a three-year, entry-level contract with the Arizona Coyotes on July 2, 2015.[2]
In the 2017–18 season, having played in the opening 7 games of the season with AHL affiliate, the Tucson Roadrunners, Mermis received his first recall by the Coyotes on November 1, 2017.[3] With an injury to fellow Coyotes defenseman Niklas Hjalmarsson, Mermis made his NHL debut the following day in a defeat to the Buffalo Sabres.[4]
Having appeared in 10 NHL games with the Coyotes during his four seasons within the organization, Mermis left as a free agent to sign a one-year, two-way $700,000 contract with the New Jersey Devils on July 1, 2019.[5] He initially joined the Binghamton Devils in the AHL to begin the 2019–20 season, leading the blueline in scoring with 19 points through 53 games. Recalled by New Jersey to the NHL, Mermis scored his first career NHL goal against the St. Louis Blues on March 6, 2020. He registered his first multi-point game the following night against the New York Rangers. Mermis appearing in 10 games with the Devils, posting four points, before their season was ended due to the COVID-19 pandemic.
Mermis left the Devils as a free agent after the season and signed to a one-year, two-way contract with the Minnesota Wild on October 9, 2020.[6]
Mermis won the Yanick Dupre Memorial Award while playing for the Wild's AHL affiliate, the Iowa Wild, in the 2021–22 season.[7]
Following four seasons within the Wild organization, Mermis left as a free agent and was signed to a one-year, $775,000 contract with the Toronto Maple Leafs on July 2, 2024.[8]