Webb first took up refereeing in local Rotherham leagues in 1989. In 1993, he progressed to the Northern Counties East League as an assistant referee, becoming a referee for that league two years later.
In 1996, he was appointed as a Football League assistant referee, and, in 1998, fulfilled the same function in the Premier League, as well as being promoted to the Football Conference as a referee. He is a police officer with South Yorkshire Police but took a five-year sabbatical leave to concentrate on his refereeing.
In 2000, he was included on the National List of Football League referees, stepping up to the Select Group for the Premier League three years later. His first game in the top tier of English football was on 18 October 2003, when he took charge of the 0–0 draw between Fulham and Wolverhampton Wanderers. He was appointed as a FIFA official in 2005.
Webb bore a one-week demotion from officiating in the Premier League down to the Football League Championship in April 2009, a punishment sporadically imposed on referees who make high-profile contentious errors. Webb had been earlier been appointed to referee the year's FA Cup final, arguably the highest domestic honour for an official, when he awarded Manchester United a debatable penalty kick while they trailed 2–0 to Tottenham Hotspur. The penalty was converted and United went on to win the match 5–2. Webb later admitted he had made "a mistake" but had made the decision "honestly".
Since then, Webb has been appointed to referee some of the world's highest-profile football matches, including an FA Cup Final, a UEFA Champions League Final and a FIFA World Cup Final.