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 although the club offered him a new deal at the end of that season, he turned this down, and he was placed on the transfer list. He was taken off that list two months later, remaining at the club for the 2007-08 season, but his shoulder problems meant that he was not able to start that 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. He was also retained for the 2008 / 09 season, in which he appeared 39 times in the Blue Square Premier, with 11 of them as substitute, including an unbroken run from the start of November until the end of the season.