Williams told Campaign that he intended to take a break from work over the summer before looking for a new role in the autumn.
Mondelez said it would announce news on a replacement next week.
Williams has worked at Mondelez and its predecessor companies, Kraft Foods and Cadbury, for 15 years. As category head for global chocolate at Cadbury, he led a project to extend the brand into bite-sized formats.
After Cadbury was acquired in 2010 by Kraft Foods – the company from which Mondelez International was later spun off – Williams became platform director, global chocolate, and later marketing activation director, UK chocolate.
He was responsible for the refresh of the Dairy Milk brand, the launch of spin-offs such as Marvellous Creations and campaigns including "Free the joy". Last year, he reintroduced the Milk Tray Man for the brand’s centenary celebrations.
Under Williams, Dairy Milk grew into a brand worth £528.5m in sales in 2015 (according to Nielsen Scantrack), compared with £371.3m in 2009, and the fourth-biggest grocery brand in the UK after Coca-Cola, Warburtons and Walkers.
