Paul Underwood signed for Diamonds in 1997 for £50,000 from Enfield. Initially playing as a left-winger Paul dropped back into the role of Left Back where he excelled, eventually becoming the Club Captain. He helped The Diamonds win the Football Conferecne and promotion to Division 3 in 2001. The following season saw him lead the team to the Division 3 playoffs, and then take the Division 3 title the following season. He continued to excel in the left back role in Division 2, before cost-cutting measures at the club forced his departure in spring 2004, and a move to Luton Town, after more than 200 Diamonds appearances.. 

Underwood established himself as a Diamonds favourite under Brian Talbot and as well as representing the England Semi-Pro team. He was named as The Diamonds player of the season in 2003. In a 2004 BBC survey Paul was voted Rushden & Diamonds all time cult hero ahead of Duane Darby and Gary Butterworth.

During his time at for Luton Town he was in their promotion winning team under Mike Newell, but after injury and a related illness he was forced to retire from the game,