McCartney played major junior hockey in the Western Hockey League (WHL) with the Brandon Wheat Kings. He played in five seasons with the Wheat Kings scoring 60 goals and 151 points in 205 games. He was selected by the Arizona Coyotes of the National Hockey League (NHL) in the seventh round, 204th overall, in the 2020 NHL Entry Draft. He signed a three-year, entry-level contract with the Coyotes on May 27, 2021.[2] Following the end of his junior year McCartney joined Arizona's American Hockey League (AHL) affiliate, the Tucson Roadrunners for the last four games of the season, making him the youngest player for the team in their history.[3]
During the 2021–22 season, McCartney scored the first of two hat tricks in a 4–0 win over the Texas Stars in the Roadrunners' home opener on October 23, 2021.[4] He was recalled by Arizona in November 2021 and made his NHL debut on November 5 in a 3–1 loss to the Anaheim Ducks.[5] He was returned to Tucson after the game.[6] He scored his second hat trick for Tucson in a 6–5 loss to the Colorado Eagles on April 16, 2022.[7] McCartney finished the season with 18 goals and 35 points in 55 games, second on the team in goals and fifth in scoring. He began the 2022–23 season with Tucson, where he was named an alternate captain.[3] He began the 2023–24 season with Tucson, but was recalled by Arizona on October 28, 2023.[8] He saw no action with Arizona and was returned to Tucson on October 30.[9]