After a 2007 ruined by a recurring hamstring injury, Mackay made his debut for Adelaide in round 1 of the 2008 AFL season, in a loss to the. He played 19 games for the season, missing only four, and kicked his first AFL goal in round 17 against. He again was a regular for the side in 2009, playing 20 matches, and by this early stage of his career was rated a vital part of the Crows' young midfield. He played 16 games in 2010. In the first round of 2011, Mackay suffered a shoulder injury and underwent surgery that kept him out for three months. He returned late in the season and showed good form, impressing both with his offensive running and defensive pressure. Mackay benefited from improved strength and durability the next season, playing 23 games and averaging 17 possessions and four tackles per game, while playing both in the midfield and across half-back. At the end of the year he signed a three-year contract extension. Much like the team as a whole, Mackay struggled for consistency in 2013. He recaptured his best form in 2014, playing 19 games, averaging 18 disposals and kicking a personal best 11 goals for the year. He signed a four-year contract extension midway through the season. Mackay started 2015 in good form, laying a career-best 13 tackles along with 23 disposals against in wet conditions in round 3. However, his old inconsistency resurfaced resulting in him being dropped late in the year. He returned to the side to play in Adelaide's two finals. At the end of 2016 he was made a life member of the Adelaide Football Club. As part of his preparation for the 2017 season, Mackay spent as much time in the backline over the pre-season as he did in the midfield due to coach Don Pyke’s emphasis on flexibility. Against in round 2, Mackay tackled Paul Puopolo to the ground in the third quarter. Though the tackle appeared fair, the pair of players continued rolling, ending up with Mackay lying on Puopolo’s back and a free kick was paid against Mackay for a push in the back. The free kick was seen as questionable and prompted divided reaction on social media. In the Showdown the next week, he kicked one of the goals of his career, snapping from the boundary line, bringing the Crows back within a goal of in a pivotal moment of the game. Mackay was dropped from several matches throughout the season and played for the reserves in the SANFL, where he was told to work on his toughness at the contest. When he came back to the AFL side, the result was he increased the number of tackles he made per game from three to five. When teammate Brodie Smith injured his anterior cruciate ligament, Mackay had to shift from his usual position on the wing to half-back to replace the former All-Australian in the finals. Mackay ended up playing in Adelaide's first grand final since 1998, which they lost to by 48 points.
