Mackail-Smith was brought through the youth system at St Albans City, scoring two goals in 30 starts and 15 substitute appearances. He was released at the end of the 2003 season and joined Arlesey Town where he played for only one season, a highlight of which was knocking Dagenham & Redbridge out of the F.A Trophy.
Mackail-Smith had a slow start to life as a Peterborough United striker, only playing rarely in his first season in league two. But from his second season through to the championship he has scored many goals. And now following the departure of McLean to recently relegated Hull City in 2010, Mackail-Smith has had his best season scoring 35 goals, the most prolific scorer in England in the 2010-11 season. On 29 May 2011 Mackail-Smith scored the second goal in the 3-0 win against Huddersfield at Old Trafford to clinch promotion for Peterborough to the Championship.