Mackey became highly coveted as an undrafted free agent after playing three years of college ice hockey for Minnesota State University, ultimately signing with the Calgary Flames in March 2020.

He was named an alternate captain in 2016–17 and emerged as an offensive engine on a Gamblers squad that also featured future NHL first-round pick Casey Mittelstadt.

[5] Prior to the start of his college career, Mackey was invited to and attended the Calgary Flames' 2017 summer prospect development camp as an undrafted free agent.

[6] Beginning that fall, Mackey enjoyed a successful debut season with the Mavericks which saw him earn a spot on the 2017–18 Western Collegiate Hockey Association (WCHA) All-Rookie Team.

[9] Despite his contract not taking effect until the following season, Mackey joined the Flames for practices in the Edmonton bubble as part of the return-to-play program in the 2020 Stanley Cup playoffs.