Youth Dermott began his youth hockey career with the Minor Midget “AAA” York Simcoe Express. During the 2010–11 season, he broke his collarbone twice causing him to only play in 19 games. During his tenure with the Express, Dermott helped the team win five OMHA championships and an OHF peewee title. Scouts witnessed Dermott's success and he was offered a try out with an East Coast select spring hockey team and signed with the
Newmarket Hurricanes in the
Ontario Junior Hockey League (OJHL). but continued to play with the Newmarket Hurricanes. After being named to the OJHL's 2nd Team All-Stars and Fan Favourite, he committed to play for the Erie Otters on May 4, 2013. During his tenure with the Otters, Dermott attended
McDowell High School and was named Midwest Division Academic Player of the Month for January 2014. Following a strong first year performance with the Erie Otters in which he scored 28 points in 67 games, Dermott was rewarded when he was named to the
2013–14 OHL First All-Rookie Team. in 2013 During
following season, Dermott scored 45 points in 61 games, including a franchise record seven-game assist streak which was the longest such stretch by a defenceman. He also led all Erie defencemen in playoff scoring with 17 points earning him a jump in ratings and a projection as a possible first round pick. Dermott attested his scoring prowess as the result of filling in for a suspended
Kurtis MacDermid and practicing with former NHL defenceman
Jay McKee. a boost from his midterm ranking of 47. Dermott was eventually drafted 34th overall by the
Toronto Maple Leafs in the
2015 NHL entry draft and signed a three-year, entry-level contract with them on July 22, 2015. Prior to his draft, Dermott began training with Dave Harris at Iperformance. On December 1, 2015, Dermott and teammate
Dylan Strome were selected to compete with Team Canada at the
2016 World Junior Ice Hockey Championships. Upon returning from Finland empty handed, Dermott was ejected from a game against the
Mississauga Steelheads for a hit to the head on
Owen Tippett. After the game concluded, he was assessed a five-game suspension following a hearing. Although the Otters qualified for the 2016 OHL Playoffs, they were swept in the
OHLs Western Conference final by the
London Knights.
Professional Toronto Maple Leafs After the Otters were swept in the Western Conference final, Dermott joined the
Toronto Marlies for the
2016 Calder Cup playoffs. He made his professional post-season debut with the Marlies during the third round of the playoffs as a replacement for
Stuart Percy before they were ultimately eliminated by the
Hershey Bears. On August 26, 2016, Dermott was selected to compete at the Maple Leafs 2016 Rookie Tournament and their 2016 training camp. After participating in both, he was returned to the Toronto Marlies for their
2016–17 season. Dermott made his AHL debut on October 16, 2016, where he recorded his first career AHL goal and assist in a win against the
Utica Comets. He concluded his rookie season with the Marlies having recorded 24 points in 59 games, earning him a second-place finish amongst Marlies defencemen in scoring. but was reassigned to the Marlies to begin the season. He played on a defensive pairing with
Timothy Liljegren and earned time on the penalty-kill. Although he was a left-handed shooter, he was moved to the right hand side. Dermott's playmaking ability earned him his first NHL recall on January 5, 2018 While with the Marlies again, Dermott was named to the 2018 AHL All-Star Classic alongside teammate
Kasperi Kapanen. Following another recall on January 15 with fellow Marlies player
Frédérik Gauthier, he was given the number 23 to wear instead of his usual 3. That night, he scored his first career NHL goal in the leafs 5–0 win against the
New York Islanders on January 31. Dermott stayed will the Leafs as they qualified for the
2018 Stanley Cup playoffs against the
Boston Bruins and recorded his first playoff goal during Game 7 to tie the game 3–3, although the Leafs ended up losing the game 7–4. Despite playing in the NHL playoff series, Dermott,
Andreas Johnsson, and Kasperi Kapanen remained eligible to compete in the
2018 Calder Cup playoffs. After the Leafs were eliminated from the playoffs, Dermott was sent to the Marlies to help them in their 2018 Calder Cup playoffs run. He ended his rookie campaign with the Leafs recording one goal and 12 assists in 37 regular season games. On the heels of his successful rookie campaign, Dermott began the
2018–19 season with the Leafs in the NHL for the first time in his career. As the youngest defenceman on the Leafs roster, Dermott played on a pairing with
Ron Hainsey with the goal of moving him to
Morgan Rielly's side. Former head coach
Mike Babcock stated that “[Dermott’s issues] are all the things that are no different than Morgan Rielly....Everyone raves about you, but you’re a dash all the time. The puck is going in your net all the time. So you have to figure that out over time. That’s what the league is all about.” He played in 24 games until teammate
William Nylander returned to the lineup following a contract dispute on December 2, 2018. Dermott was loaned to the Marlies temporarily to make room for Nylander in the Leafs lineup but returned for their following game against the
Buffalo Sabres. Upon returning, he continued to play with the Leafs until February 27, 2019, when he suffered a shoulder injury. As a result, he was out of the lineup for a month before eventually returning to finish the regular season and playoffs with the Leafs. He underwent shoulder surgery during the offseason after the Leafs were eliminated in the first round the
2019 Stanley Cup playoffs. Although the
2019–20 season would eventually be delayed due to
COVID-19, Dermott began the season in late October as he recovered from his surgery in May. In the final year of his contract, Dermott began seeing playing time on the top defenceman pairing after Morgan Rielly and
Jake Muzzin suffered injuries. Following the hiring of
Sheldon Keefe, who Dermott played under with the Marlies, he was paired with
Justin Holl to replace those on the top pair. Although he was again shortly sidelined due to an illness, he only missed one game to recover. Under the guidance of Keefe, Dermott averaged nearly 19 minutes of ice time per game playing beside Holl and helped the team outscore opponents 4–1. On October 23, 2020, Dermott signed a one-year, $874,125 contract extension with the Maple Leafs. Dermott played in 51 games with the Leafs that year, scoring two goals and six points but saw his ice time diminish from previous seasons. On July 8, 2021, Dermott signed a two-year, $3 million extension with the Maple Leafs. but the
Seattle Kraken chose to select
Jared McCann from the Leafs. During the
2021–22 season Dermott played in 43 games with the Maple Leafs scoring one goal and five points. During the season he was surpassed by
Rasmus Sandin and
Timothy Liljegren on the Maple Leafs depth chart.
Vancouver Canucks On March 20, 2022, one day before the trade deadline, Dermott was traded by the Maple Leafs to the
Vancouver Canucks, in exchange for a
2022 third-round draft pick. During the
2022–23 season Dermott suffered a concussion during a practice on September 27, 2022 and remained out of the lineup until December 29. He played in 11 games with Vancouver before suffering another injury that saw him placed on injured reserve.
Arizona Coyotes On July 11, 2023, Dermott was signed to a one-year, two-way contract with the
Arizona Coyotes for the
2023–24 season. The contract carried an NHL salary of $800,000 and an AHL salary of $450,000. Dermott attracted attention shortly into his tenure with the Coyotes when, during the team's October 21 home opener game, he became the first NHL player to defy the league's new ban on the use of
Pride-themed
hockey tape. The NHL, NHL Player's Association, and NHL Player Inclusion Coalition reviewed Dermott's action, and ultimately, the ban on Pride tape was rescinded. Dermott had used pride tape regularly since he'd been in the AHL. He had only waited until a new shipment had arrived after misplacing his supply during his move from Vancouver.
Edmonton Oilers and Minnesota Wild Having concluded his tenure with the Coyotes, Dermott as a free agent was un-signed over the summer. On September 14, 2024, Dermott accepted an invitation to attend the
Edmonton Oilers training camp for the season on a
professional tryout (PTO). Dermott had a successful training camp and his PTO was converted into a one-year, two-way contract with the Oilers for the
2024–25 season. As a depth defenceman, Dermott went scoreless through 10 appearances with the Oilers before he was placed on waivers and claimed by the
Minnesota Wild on December 13, 2024. He was reclaimed by the Oilers on February 26, 2025 and was sent down to the Bakersfield Condors.
Later career On 25 November 2025,
Sportsnet's
Elliotte Friedman reported that Dermott underwent left shoulder surgery on July 23, and was looking to resume his career. On 7 February 2026, Dermott signed a
professional tryout (PTO) with the
Hartford Wolf Pack of the
American Hockey League (AHL). ==Personal life==