McMillan first competed in saloon racing in 2006, competing in the 750 Motor Club's SAXMAX Championship. McMillan dominated the championship starting every one of the ten races from pole, recording eight fastest laps and scoring nine wins to win by over fifty points in the championship standings. The only blip in the win tally was at the penultimate round at
Snetterton, when he finished in eighth position. In 2007, McMillan stepped up to the
SEAT Cupra Championship, driving in the New Leon Cupra Class. His best result of the year was a sixth place in the rain-shortened round eleven at
Donington Park. and finished the season in twelfth place from a field of thirteen runners. He also entered three rounds of the
Spanish SEAT Leon Supercopa, scoring a solitary point. For 2008, McMillan competed a full season in the SEAT Leon Supercopa with the Triple R Team and ended the season tenth on points, including three podium finishes – all coming at the triple-header
Circuit de Catalunya season finale. This was combined with four rounds entered in the inaugural
SEAT León Eurocup, scoring four points and a best finish of sixth in the opening race at
Circuit de Valencia. In 2009, McMillan competed in the
British Touring Car Championship in a self prepared ex-works
SEAT Toledo for his independent family-run Maxtreme Team, which was managed by his father Paul. However, the team never got its engine back after sending it for a refit, forcing them out of the series after just three rounds. ==Racing record==