The firm wanted a fresh look at its comms approach following appointments of Lyndsay Wright as William Hill's first head of investor relations six months ago and Ralph Topping as chief executive last year.
Industry sources initially speculated the firm may have wanted a new agency in light of a brief to help drive the company in a new direction.
Wright met 15 agencies before narrowing them down to a four-strong shortlist of Brunswick, College Hill, FD and Tulchan Communications.
It is understood the pitch came down to a two-way shoot-out between College Hill and Brunswick.
Wright said: 'We wanted a new set of eyes on what we are doing as a business and what we should be saying to the market. I am sure Brunswick will bring that.'
Although staying within Brunswick, the account will be run by an entirely different team, now led by partner Simon Sporborg.