An experienced player, his career started in 1992 at Colchester United, where he made 37 league starts between July 1992 and October 1993, before moving on to Brentford on October 21 for a fee of £60,000. He was a first-team regular throughout his time at the club, making 100 league starts and scoring 12 goals. He joined Birmingham City on March 25, 1996, and quickly became an inspirational player due to his continued consistency and ability from set-pieces.
He picked up the fans' Player of the Season award for 1999-2000, but as Birmingham continued to improve in the First Division, and eventually earned promotion into the Premiership, Grainger's playing time became limited. He failed to appear for Birmingham during the first half of the 2003/04 season, and in February he was loaned to Coventry City, for whom he made seven league appearances. On March 18, 2004, he was recalled to Birmingham's team due to an injury crisis, and got on the scoresheet in a 2-1 defeat to Manchester United, but was himself injured and failed to appear in any matches for the rest of the season.
Grainger retired from football on January 1, 2005, having spent 13 years as a player. His goal-scoring appearance against Manchester United, on April 10, 2004, proved to be his final professional fixture. In the years following his retirement, Grainger has continued to be a popular figure amongst supporters of Birmingham City. He currently works as a VIP chauffeur.
On the 10th January 2008, Cheshunt F.C. issued a statement saying Martin would be the new Cheshunt F.C. manager, but on the 12th January 2008, another statement was issued, stating that he had left the club for family and personal reasons, wishing him all the best. He is currently the shortest-reigning manager of Cheshunt F.C..