At the same, Katie Myler is quitting her role as senior media adviser to the new Labour leader Ed Miliband. She will join Burson-Marsteller as director and senior consultant in corporate business.
Flanagan will oversee a new digital division at Portland when he joins this month. He joined Downing Street in 2008 with a brief to overhaul Brown's lacklustre web strategy, but was quickly promoted to head of strategic comms.
As a civil servant, Flanagan remained in place once David Cameron entered Downing Street and has spent the past months working closely with Cameron's strategy chief, Steve Hilton, before leaving this week. He insisted that his departure was not linked to the change of Government.
Myler, meanwhile, will work under B-M's corporate lead Chris Cartwright. She told PRWeek she had set her sights on a senior corp comms role soon after the general election and had been in negotiations with WPP and B-M since the summer.
A former special adviser to two Labour secretaries of state, Myler played a key role in Miliband's successful leadership campaign, acting his press handler. Before moving into political comms, she spent a decade at GMTV as a senior producer and Cartwright said her skills as a senior media strategist would prove valuable in the corporate arena.
The appointments come as Labour makes a concerted effort to snare an experienced journalist to be its comms director.
The party is now said to be actively searching for a journalist with a television background to oversee media and campaigns. Labour sources told PRWeek at least one senior broadcast journalist had been approached by Labour to take the vacant top comms role.
- Katie Myler previously worked as an adviser to then home secretary Jacqui Smith and ex-secretary of state for health Andy Burnham. Before that, she spent ten years at GMTV
- Mark Flanagan was a new media adviser to Tony Blair's Government before joining the civil service to overhaul No 10's digital comms. Before that, he had a 20-year career in radio.