After spending the 2003 AFL season in the Geelong reserves further developing his game, Mackie made his senior debut in 2004, and picked up a Rising Star Nomination for a three-goal performance. Despite having played as a midfielder for Glenelg, Geelong attempted to turn Mackie into a key forward to fill a long-standing void, with mixed results.
In the 2007 season Mackie began to show great improvement in his game and was one of the key players to Geelong's rise to the top. Mackie began to play as a rebounding defender, ironically kicking a career-best four goals against Richmond in Round 6 in one of his first games down back, and played another great game in Round 11 2007 when he played his first ever game against Adelaide at AAMI Stadium. He racked up 26 disposals helping Geelong to an important win and played well against Adelaide again in Round 19. He continued his solid form for the rest of the season and played a big part in Geelong's hard fought preliminary final win over Collingwood. Mackie was a key player in the 2007 premiership team, setting up play for the Cats and kicking a goal.
In 2005, a late night incident involving Mackie, his club teammate Steve Johnson and the police was reported by the media. Out celebrating Mackie's 21st birthday, Mackie and Johnson were involved in a heated discussion with local police whilst waiting for a taxi in the early hours of the morning. Police charged both Mackie and Johnson with being drunk in a public place, and sent both back to the Geelong police station. Mackie was released after the charges were laid, while Johnson was detained for a further four hours.
He is a part-owner of a thoroughbred racehorse with former teammate Brad Ottens.