MSL Group announced the move this morning in a short statement that confirmed Williams' departure "to pursue another opportunity".
The statement went on: "We wish him well and thank him for his 12 years at Capital MSL and within MSL Group.
"Co-founder Richard Campbell continues in his role as chief executive of the firm, supported by a strong and broad management team, who have successfully presided over industry-leading growth in recent years."
Williams, Campbell and Nick Lockwood (who has since left the agency) founded Capital with backing from Publicis in 2001 and have grown the agency into a top ten player in the UK City PR market.
Campbell said that the agency would not look to replace Williams directly, but added: "We see this as an opportunity to bring in new ideas and networks."
Capital has largely built its business on growth in cross-border international work, particularly the Middle East.
The merger of WPP-owned UK City PR giant Finsbury and US corporate PR business RLM in January 2011 expanded Finsbury’s international reach. It is thought Williams’ role is likely to include aiding the continued development of this international network.
Williams is understood to have begun a period of gardening leave before assuming his new post at some point in early 2014.