Known as “B-Mack,” he first stepped onto skis at just two years old, following in the footsteps of his avid skiing parents. At age four, he joined the Nancy Greene ski program at Nakiska and spent several years competing in alpine ski racing. At 10, inspired by his older brother's transition to freestyle and freeride skiing, Mackay followed suit, joining the freestyle club at Canada Olympic Park and the freeride club at Lake Louise. After exploring a variety of disciplines, including moguls and big mountain skiing, he eventually found his focus in halfpipe. He joined Canada's National Team in 2015.

Mackay has established himself as one of the world's top halfpipe skiers, earning 11 FIS World Cup podiums, including three victories. One of the defining moments of his career came at the 2023 FIS World Championships in Bakuriani, Georgia, where he captured gold in men's halfpipe to become world champion. He has also earned a bronze medal at the 2020 X Games and finished ninth at the Beijing 2022 Olympic Winter Games. At the 2026 Milano Cortina Olympic Winter Games, Mackay reached the Olympic podium for the first time, capturing bronze in men's halfpipe.