After showing great promise and skill in the youth set-up and reserve team, local lad Marcus was named on the bench for the Rushden & Diamonds Division 2 matches against Bournemouth, Stockport County and Barnsley in the 2003-04 season and soon made his full debut against Brighton & Hove Albion away. In the 2004-05 season, Marcus made a further three starting appearances plus eight as substitute, and in the summer of 2005 was rewarded with a six month pro-contract. Following a string of fine performances at the beginning of the 2005-06 campaign, which saw him become a first team regular, Marcus's contract was soon renegotiated by the club.

Shoulder problems which required an operation reduced his opportunities during the 2006 / 07 season,  and meant that he was not able to start the 2007 / 08 season, but he made it back by mid-September, and quickly re-established his place in the Diamonds side. His season was especially memorable for the goal he scored at the Memorial Stadium against Bristol Rovers in the FA Cup - an effort from almost on the half-way line which earned special praise from Alan Shearer on Match of the Day.

Further injury meant he missed the end of the 2007 / 08 season after helping his side to the Setanta Shield final.

Kelly in action in the 2007 / 08 season

Kelly in action the 2006 / 07 season

In the FA Youth Cup, Marcus takes on Manchester United