Wililamson started his career with Mansfield Town, making over 150 appearances in five years with the Stags before signing for Northampton Town in September 2004.
A spell with Rotherham United soon followed, before he made the move to Premier League club Watford in January 2007. Williamson would go onto play over 75 games during his two-and-a-half years with the Hornets.
Sheffield United secured the midfielder's services in the summer of 2009, where he would go on to score an impressive 19 goals in 89 appearances.
Former manager Michael Appleton brought Williamson to Portsmouth on a short-term deal, before returning to secure his services with Rovers.
Strong displays in the centre of midfield saw Williamson earn a new contract at Ewood Park until the summer of 2015.