Westwood made his senior debut for Warwickshire in a friendly List-A match in 2001. It wasn't until 2003 till he made his First-Class debut. For the next two seasons he was in and out of the side but cemented his place at the end of the 2005 season with some consistent performances. The 2006 season brought Westwood his maiden First-Class 100.
In November 2008, Westwood was appointed Warwickshire captain in succession to Darren Maddy.