Sheehan will join Portland PR, becoming an associate partner at the agency.
Sheehan has spent eight of the last ten years at No 10 in a range of media and strategy roles.
He has worked with former directors of comms Alastair Campbell and David Hill, as well as with current statregy chief Stephen Carter.
Sheehan was headhunted to return to Downing Street when Gordon Brown became prime minister last June.
He oversees the Downing Street 'grid'. He also led the digital and direct communications units.
Portland is run by founder Tim Allan who was Alastair Campbell’s deputy.
Sheehan follows in the footsteps of former Tony Blair speechwriter David Bradshaw and Steve Morris,who both joined Portland this year.
Last week, PRWeek revealed the chief speechwriter Beth Russel was leaving 'Team Brown'.