Mercer is a big part of the investment the New Jersey Devils made in their future when they traded Taylor Hall, the 2017-18 Hart Trophy winner voted as NHL most valuable player, to the Arizona Coyotes on Dec. 16, 2019, for the right to take the Carbonear, Newfoundland, native with the No. 18 pick in the 2020 NHL Draft. He signed a three-year, $12 million contract with the Devils on Sept. 20, 2024, after completing his third NHL season with 33 points (20 goals, 13 assists) in 82 games.

Mercer broke Scott Gomez's Devils/Colorado Rockies/Kansas City Scouts record when he played his 141st consecutive game to start an NHL career, scoring twice including 2:30 into overtime of a 4-3 win against the Los Angeles Kings at Prudential Center on Feb. 25, 2023. He got his 100th NHL point (a goal) in a 4-2 home loss to the Washington Capitals on Nov. 10, 2023, and scored twice in his 200th NHL game, a 6-3 win at the Capitals on Jan. 3, 2024. His first NHL hat trick came in a 5-1 win against the Pittsburgh Penguins in Newark, New Jersey, on April 4, 2023.

Mercer completed the 2021-22 season as the only Devils skater to play all 82 games. He scored his first NHL goal against the Seattle Kraken on Oct. 19, becoming the first New Jersey player to score against the 2021 expansion team, and is the first player in franchise history to wear No. 91.

Mercer has a decorated international resume headlined when he helped Canada to a gold medal at the 2020 IIHF World Junior Championship and silver the following year. He played four seasons in the Quebec Major Junior Hockey League, with Drummondville and Chicoutimi, leading the latter as an assistant captain with 36 points (19 goals, 17 assists) in 2020-21.