He will add the duties to his day job as chief executive of City PR agency Brunswick and a non-executive directorship of builders merchant Travis Perkins.
Carter was chief executive of media and telecoms regulator Ofcom between 2003 and 2007. Prior to that he was managing director of debt-laden cable operator NTL.
His experience working for a regulator will probably invaluable as Royal Mail lobbies Postcomm on the issue of deregulating bulk mail services. Traditionally, Royal Mail has had a fractious relationship with the regulator.
Allan Leighton, chairman of Royal Mail, said of Carter's appointment: "He brings a wealth of experience from the worlds of telecoms, marketing and regulation to the board at a time when we are facing increasing competition from a range of other communications media as well as rival postal operators."