The company hired the agency after a final two-way pitch against agency MCBD.
William Hill selected The Brooklyn Bothers to handle its UK advertising account, worth an estimated £5m, as it prepares for the 2010 FIFA World Cup.
Previously, William Hill worked with Manchester agency BJL. It created a TV push for its online casino gaming in 2008, following a pitch.
The brand spent £5m on advertising in the UK in 2008, according to Nielsen.
The company has also entered into phase two of its brand refresh, which is being handled by Wolff Olins.
In June, William Hill appointed the branding agency to overhaul its brand positioning to respond to changing consumer betting habits.
At the time, Jarrod Hollingdrake, William Hill's marketing manager, said Wolff Olins had been hired on a short-term basis to work on proposals looking at the company's brand strategy.