Mackenzie has also played for Hereford United and even scored a goal for them. The goal was scored against Barnet on the opening day of the 1995 - 96 season and the conceding goalkeeper was none other than Maik Taylor, who was making his league début on that very day. As well as Shrewsbury and Hereford and several non-league clubs, Mackenzie has played in the Football League for Leyton Orient and Chester City.
Mackenzie signed for Shrewsbury on 15 May 2006 on the Bosman ruling from Chester City, where he had spent two years as predominantly first choice goalkeeper. He started the season as second choice goalkeeper behind Ryan Esson, but was handed his club début on 22 August 2006 in the League Cup match at Birmingham City, which Town lost 1 - 0. By mid-September Mackenzie had gained a spot in the starting line-up, and shortly afterwards, Esson suffered an injury in a reserve fixture, giving Mackenzie a chance to make an extended run in the team.
As Esson continued to struggle, Mackenzie found himself competing with loanee Scott Shearer for a place in the starting eleven. In the closing month of the season Shearer was being picked ahead of Mackenzie but (due to an agreement made prior to his loan signing) was not allowed to play against his parent-club Bristol Rovers in the League Two play-off final. At the age of 35, MacKenzie subsequently got his chance to line up at Wembley for the first time, though Shrewsbury succumbed to a 3 - 1 defeat.
On 21 September 2007, Mackenzie joined Conference side Kidderminster Harriers on a month long loan to provide cover for the injured Scott Bevan. Despite having an bad début at Aggborough where he got lobbed from 40 yards this was later extended by a further month, and he returned to Shrewsbury in November. However, Mackenzie made the permanent move to Kidderminster on 31 January 2008, as part of a deal that saw Bevan move in the opposite direction.
At the end of the 2007 - 08 season he was released by Kidderminster Harriers. Mackenzie then signed for Hinckley United in May 2008.
He rejoined his first club, Corby Town in May 2010, before signing for Alfreton Town in 2011.