As reported on prweek.com (1 July), Sheehan will join Portland PR as an associate partner.
Last week, PRWeek revealed that chief speechwriter Beth Russell was leaving 'Team Brown' after nearly a decade.
Other recent exits from Number 10 include political secretary Fiona Gordon, who is setting up her own consultancy service, and senior press officer Jo Millington, who has left to join the Department of Transport. Political strategy director Spencer Livermore left earlier this year.
One insider said: 'We haven't seen the last of the departures.'
Sheehan denied that his departure was related to other recent high-profile Downing Street exits and refused to comment on the mood within Brown's comms team. He said he was leaving because 'it's time for a new challenge'.
Sheehan has spent eight of the past ten years at Number 10 in a range of media and strategy roles.
The Strategic Communications Unit (SCU), which Sheehan currently heads, is a vital cog in the Downing Street comms machine, focusing on long-term comms strategy.
As such, it is one step removed from the daily media battles fought by the Downing Street press office and by Brown's top media handler Damian McBride.
Former Which? comms chief Nick Stace was recently signed up to help run the unit alongside Sheehan.
As well as Sheehan, the SCU is also home to another respected survivor from the old regime, Paul Brown.
Portland PR is run by Tim Allan, a former deputy to Alastair Campbell at Number 10.