Playing career
Marchment did not start his ice hockey career until the age of 16, playing two years of minor ice hockey with the Central Ontario Wolves U18 AAA team from 2011 to 2013. The following year, he played with the Cobourg Cougars of the Ontario Junior Hockey League (OJHL).
After going undrafted the previous three years of eligibility, Marchment made his junior ice hockey debut with the Erie Otters of the Ontario Hockey League (OHL) at age 19 in the 2014–15 season. Marchment was traded at the beginning of the 2015–16 season, and would play that season with the Hamilton Bulldogs and then the Mississauga Steelheads. He recorded 77 points (28 goals, 49 assists) in 115 career OHL regular-season games.
Marchment was signed by the Toronto Maple Leafs to an American Hockey League (AHL) contract on April 8, 2016, and would make his AHL debut with the Toronto Marlies in the final games of the 2015–16 season. He split his 2016–17 season between the Marlies and the Orlando Solar Bears of the ECHL minor professional team, playing most of the season with the ECHL.
Marchment spent his first full season in the AHL in the 2017–18 season, and signed a two-year, entry-level contract with the Maple Leafs on March 17, 2018. During the 2018 Calder Cup playoffs, Marchment recorded nine points over 20 games, and scored the winning goal in game 7 of the finals to win the Calder Cup for the Toronto Marlies. Marchment continued to play in the AHL during the following season, and recorded seven points in 13 games during the 2019 playoffs, where the Marlies would lose in the conference finals.
During the 2019–20 season, on January 1, 2020, after losing several forwards to injury, the Maple Leafs called up Marchment to make his NHL debut the following night against the Winnipeg Jets. He registered one assist in four appearances with the Maple Leafs, across multiple callups before he was traded by Toronto to the Florida Panthers in exchange for Denis Malgin on February 19. Marchment recorded 86 points (48 goals, 38 assists) in 157 career AHL games with the Toronto Marlies. He also recorded two assists over six games with the Springfield Thunderbirds, the AHL affiliate of the Panthers, before the COVID-19 shutdown, which lasted until February 2021.
On November 3, 2020, Marchment as a restricted free agent was re-signed by the Panthers to a one-year, two-way contract. On January 13, 2021, he was placed on the Panthers' taxi squad, and was activated for one NHL game on February 7 before being placed back on the taxi squad. He was then re-activated on February 25, and remained on the active roster through the end of the season. Marchment scored his first NHL goal on March 5, in a 5–4 win against the Nashville Predators, ending the season with 10 points over 33 regular-season games and two points over six playoff games.
On April 5, Marchment was signed to a one-year contract extension by the Panthers. On January 31, 2022, Marchment recorded a six-point (two goals and four assists) game, tying Olli Jokinen's franchise record for most points in a game, in the Panthers' 8–4 win over the Columbus Blue Jackets. On February 18, Marchment scored his first career hat-trick in a 6–2 win over the Minnesota Wild. Marchment faced multiple injuries throughout the 2021–22 season, finishing with 47 points (18 goals, 29 assists) in 54 regular season games and one goal in four playoff games.
As a free agent from the Panthers, Marchment was signed to a four-year, $18 million contract with the Dallas Stars on July 13, 2022. At the start of the 2022–23 season, Marchment made the NHL opening night roster for the first time in his career, and made his debut with the Stars in the season opener on October 13. He recorded two goals to lead his team to a 4–1 win over the Nashville Predators. Marchment suffered a knee injury on March 13, 2023, and missed 14 games, returning to the lineup on April 13. He finished the season with 31 points (12 goals, 19 assists) in 68 regular-season games and six points (four goals, two assists) in 18 playoff games. Marchment also recorded 80 penalty minutes, ranking first on the team and more than 20 minutes more than the next player.
On February 8, 2024, Marchment was fined $5,000, the maximum penalty allowed by the Collective Bargaining Agreement, for a late board-hit during the game the previous night against the Toronto Maple Leafs. Marchment finished the 2023–24 season with career-highs in points, goals, and assists, recording 53 points (22 goals, 31 assists) in 81 regular-season games, and five points (three goals, two assists) in 13 playoff games. He also recorded 54 penalty minutes, once again ranking first on the team, with 13 minutes more than the next player.
On December 27, 2024, Marchment was hit in the face by a deflected puck in a game against the Minnesota Wild and had to be taken to the hospital. He required facial surgery, and missed 17 games, returning to the lineup on February 2, 2025. Marchment would conclude the 2024–25 season with 47 points (22 goals, 25 assists) in 62 regular-season games and five points (one goal, four assists) in 18 playoff games.
On June 19, 2025, the Stars traded Marchment to the Seattle Kraken in exchange for a 2025 fourth-round pick and a 2026 third-round pick. Marchment played 29 games registering 13 points with the Kraken, before being traded on December 19, 2025, to the Columbus Blue Jackets in exchange for 2026 fourth-round pick and a 2027 second-round pick.
Marchment scored a goal in his Columbus debut on December 20, and went on to record six points in his first five games with the franchise, his longest point streak. On January 24, 2026, Marchment had his first four-point game, which included a hat trick in an 8–3 win against the Tampa Bay Lightning. On March 17, he scored his 12th goal with the franchise, breaking the record for most goals by a Columbus player in their first 25 games. With the Blue Jackets, Marchment registered 32 points (15 goals, 17 assists) in 39 games and had a career-high shot percentage of 20.5%.
A free agent at the end of the 2025–26 season, Marchment signed a five-year, $33.75 million contract with the San Jose Sharks on July 1, 2026. Marchment made his Sharks debut in the 2026–27 season opener on October 1, against the Florida Panthers. His first goal as a Shark was also the team's first goal of the season, and he and his late father, Bryan Marchment, became the second father–son duo to score for the Sharks.
