Lee Tomlin was signed as a sixteen year old in January 2005 by Rushden & Diamonds following his release from the Leicester City youth scheme, and in October of that year he made Diamonds history as the Club's youngest ever player when he came on as a substitute in an away game at Wycombe Wanderers. After a number of substitute appearances he made his full debut for the club at Nene Park in the First Round FA Cup Replay against Halifax Town in November 2005, and his full league debut against Peterborough United while he was still aged 16.

In the 2006/07 season he scored his debut goal away at Weymouth on 21st October 2006 - and at 17 years 9 months and 9 days that made him the youngest ever league goal scorer for the club.

The following season saw him demonstrate his versatility by playing in wide midfield as well as upfront, making 42 appearances for the club during that campaign, and on 17th June 2008 he signed a new contract until the end of the 2010/11 season.

In his Rushden & Diamonds career prior to the 2008-09 season he had appeared in 107 games, many as substitute, scoring 15 goals.

During the 2008-09 season he scored 8 league goals, making him the joint-top league goal scorer that season. He played much of that season in a wide-right role, but was given a more conventional striking role when Justin Edinburgh took charge of the Diamonds. His efforts and commitment meant that he also won the Players' Player of the Season award for that season.

He appeared for the England C team during that season in the 4-0 victory against Malta, scoring once and being awarded the Man-of-the-Match. He was rewarded with a further call-up for the England C team in a post-season game.