The company, whose account was previously handled by the Leeds-based Poulters, initially approached agencies about a review in April 2007.
However, it did not progress to a formal pitch, and William Hill instead opted to move the business directly into Burnett.
The agency, which has already been working with William Hill on a brand consultancy basis, will be briefed to create a campaign designed to introduce a new consumer brand positioning.
Following the implementation of the Gambling Act last year, the campaign will be heavily skewed towards television work.
Chris Edgington, the group marketing director at William Hill, said: "2008 is going to be a big year for William Hill. Leo Burnett has already partnered with us to develop an exciting new positioning for the brand that will enable us to compete more strongly in the growing online market and secure our position as the leading betting brand in the UK."