Grainger had been at Brands2Life, the UK's 30th largest PR agency according to PRWeek's Top 150, for 10 years, and held his previous role since 2012. Before that he had worked at Fuse PR and Marlin PR.
In his new role, Grainger will "lead the development of an international capability that will help clients enhance their communications by drawing on complementary expertise within the agency", a Ruder Finn release said.
He will also "identify and drive opportunities for growth across the global Ruder Finn network", and report to Ruder Finn UK MD Nick Leonard.
Leonard, who took up his post in 2008, said: "Organisations today are increasingly looking for guidance on scaling their communications globally, delivering a consistent message to internal and external stakeholders, and unlocking new market sectors. Ruder Finn is perfectly placed to capitalise on this trend, and Robin will be instrumental in charting this new stage of our development."
With revenues of $73.9m (£51.8m) in 2014, Ruder Finn is the 25th largest comms firm in the world according to PRWeek's 2015 Global Agency Business Report, and the agency says its UK team has grown by 50 per cent in the past three years.
It has owned offices in New York, San Francisco, Washington DC, Boston, London, Paris, Basel, Beijing, Hong Kong, Shanghai, Singapore and Guangzhou, according to its website, with dual headquarters in the US and China.