Duncan will become the consultancy's fifth partner and will focus on advising private equity companies on potential mergers and acquisitions in the industry. The advertising sector has become more popular again with private equity companies as marketing budgets have grown.
Grace Blue was founded by Gay and Jay Haines, formerly of Kendall Tarrant; Juliet Timms, the ex-head of Bartle Bogle Hegarty's now defunct direct operation, Limbo; and Kathleen Saxton, the former Virgin Radio head of strategy.
Duncan said: "My career to date has been spent running big, established ad agencies. For the next stage, I wanted to help create something from the ground up, along the way capitalising on 25 years of experience and contacts in the communications business." Timms added: "We're very fortunate to have someone of Grant's calibre at Grace Blue. He completes our line-up of partners."
Grace Blue was founded last year with an ambition to place senior executives in marketing and advertising positions.
Duncan resigned from the chief executive's position at Publicis in February after an eight-year stint at the creative agency.