Playing career
Maccelli began his junior career with HC TPS of the Finnish Liiga. In 2017, he joined the Dubuque Fighting Saints of the United States Hockey League (USHL). In 98 games with Dubuque, he recorded 39 goals and 54 assists for 93 points. In his second year in the USHL, his play led him to being named to the ALL-USHL Second Team. In 2019, Maccelli returned to Finland to join Ilves of Liiga.
Maccelli was selected 98th overall in the fourth round of the 2019 NHL entry draft by the Arizona Coyotes. In his first season with Ilves, in the 2019–20 season, he recorded 13 goals and 17 assists for 30 points in 42 games. Maccelli signed a three-year, entry-level contract with Arizona on April 29, 2020, while still playing for Ilves. He was named the league's rookie of the year, being awarded the Jarmo Wasama Memorial Trophy. On July 17, it was announced that Maccelli would stay with Ilves after an open-ended agreement was reached between the Finnish team and Arizona. He played well with Ilves during the 2020–21 season.
For the 2021–22 season, Maccelli returned to North America, joining Arizona's American Hockey League (AHL) affiliate, the Tucson Roadrunners. He recorded 14 goals and 43 assists for 57 points in 47 games with Tucson. He was recalled by Arizona in March 2022 to replace Andrew Ladd who had been placed on injured reserve. He made his NHL debut in a 2–1 win over the Colorado Avalanche on March 3. Maccelli scored his first NHL goal in his second game on March 6, versus the Ottawa Senators.
Maccelli broke out in the 2022–23 season, making the Coyotes' roster out of training camp. In his first 30 games, he recorded three goals and 19 assists for 22 points. He suffered a lower-body injury that led to some missed time, but upon his return, scored seven goals and 16 assist for 23 points in his next 30 games. He finished the season with 11 goals and 38 assist for 49 points in 64 games with the Coyotes and was named to the NHL's All-Rookie Team, playing a major role in the team's power play. On July 16, 2023, Maccelli signed a three-year contract with Arizona.
Shortly after the end of the 2023–24 season, the Coyotes' franchise was suspended and team assets were subsequently transferred to the expansion team Utah Hockey Club; as a result, Maccelli became a member of the Utah team.
Following the 2024–25 season, Utah traded Maccelli to the Toronto Maple Leafs on June 30, 2025, in exchange for a conditional third-round draft pick in 2027, which could have become a second-round draft back in 2029 if the Maple Leafs qualified for the 2026 Stanley Cup playoffs and Maccelli scored 51 points (neither condition met). Acquired by the Maple Leafs as a potential Mitch Marner replacement and in the hopes that he would bounce-back to his form in previous seasons with the Coyotes, Maccelli had a disappointing campaign with the Maple Leafs relative to expectations, with Macceli playing up and down the lineup throughout the year and occasionally being a healthy scratch. Despite this, he recorded 39 points with the Maple Leafs. Upon expiration of his contract, he was not tendered a qualifying offer by the team and subsequently became an unrestricted free agent.
On July 1, 2026, Maccelli signed a one-year, $2.25 million contract with the New York Islanders.
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