Fahy, who joined the company at the beginning of the year, is understood to be planning to put greater emphasis on digital marketing.
He has also put the bookmaker's £5m advertising account up for review. It is believed that The Brooklyn Brothers, which was hired in November, will lose the work.
The agency recently completed a project in association with ITV to promote William Hill's betting products during the latter stages of the FA Cup and Champions League.
According to Mintel, William Hill is the biggest chain of bookmakers in the UK.