Mackenzie was drafted in the 2nd round of the 2006 AFL Draft.[1]
A key position defender, he had his best season to date in 2014. He replaced the retiring Darren Glass as the club's leading backman and was named acting co-captain of the club for the remainder of the season, along with four other players.[2] He ended the year by winning the Eagles' best and fairest award, the John Worsfold Medal, beating Brownlow MedallistMatt Priddis by 11 votes.[3]
In the final minutes of the 2017 Elimination Final played against Port Adelaide Power, with scores level, Mackenzie kept Port Adelaide from scoring a behind, throwing the full force of his body into the point post in the process.[5] This helped force the Elimination Final into extra time, where the Eagles emerged victorious.
Mackenzie retired at the end of the 2018 season, after consistent troubles with injury.[6]