The agency has promoted Nereda Merrin from account director to group account director, and Wendy Tring and Chrissy Dransfield to account directors.
Claire Watson, a former account director at MTV Networks International, has joined as group account director.
Merrin and Watson will be responsible for strategic development, financial management and campaign implementation for the agency's core clients, including Unilever, Cadbury, Sky, Transport for London and Sony.
Dransfield will oversee all of RPM's Diageo projects, including the experiential work for Smirnoff and Johnnie Walker global work.
The agency said Robertson joining the board and the creation of two new group account director roles were in response to recent growth at the company, and follow the appointment of the agency's first chair, Graham Kemp, in October last year.