Bristol MD Chris Davies has taken control of Strategy Wales, Grayling's Welsh PA function, following Roberts' departure for personal reasons.
Davies joined Grayling in 1997 after a spell as head of corporate affairs for the UK arm of pharma company, Bristol-Myers Squibb UK. From 1993 to 1996 he was director of public affairs at airports operator BAA.
Davies hopes running both offices will enable him to integrate resources and strengthen the joint public affairs and PR focus of the firm, he said.
Strategy Wales was launched in 1999 to advise firms on Welsh Assembly issues. Clients include the Welsh Development Agency.