McKenzie began his career with his local club Crystal Palace, making his debut during the 1995–96 season.
He joined Norwich from Peterborough in December 2003 for a fee of £325,000, and instantly won over the Canaries supporters by scoring twice on his debut to help beat local rivals Ipswich Town at Portman Road and go top of the First Division.
His goals made a significant contribution to Norwich's promotion to the Premier League as First Division champions at the end of the 2003–04 season.
McKenzie took time to find his feet in the Premiership but he established a strike partnership with Dean Ashton which would become one of the most potent in the division during the closing stages of the 2004–05 season.
He came third in the Norwich City Supporters Player of The Season vote for the 2004–05, beaten by Darren Huckerby and Damien Francis.
[citation needed] The 2005–06 season for McKenzie was interrupted by injuries and personal difficulties surrounding the break-up of his marriage.
McKenzie was signed by Coventry City on transfer deadline day in August 2006 for an initial £600,000 that, depending on appearances and promotion, could rise to £1m.
Leon is a head coach at 12x3 gym in Aldgate East and speaks internationally about mental health.
McKenzie made his boxing debut on 29 June 2013 when he fought John Mason at York Hall, Bethnal Green in London.
The contest was brought to a halt at the end of the sixth round after McCallum was pulled out by his corner after suffering a badly damaged left eye.
On 12 November 2016 McKenzie fought Jahmaine Smyle at York Hall for the English super-middleweight title, but lost for the first time in his career.