In the newly created role of director of communications, Sellgren will steer a 14-strong team with responsibility for press relations, campaigns and internal comms. The team will also support the council's consultation programmes.
Ten further comms staff are placed within the individual departments.
He joins after seven years at the County Councils Network – part of the Local Government Association. At CCN his brief was to promote the interests of 37 'shire' counties to government, in particular to the Treasury and the Office of the Deputy Prime Minister.
'There are huge opportunities in local government at the moment in terms of the devolution agenda, which is why I want to return to a post on a local level,' Sellgren told PRWeek.
He fills the vacancy left by former head of communications James Odling-Smee, who left Hertfordshire Council at the end of 2005 to go freelance after five years in the post.
Hertfordshire Council, which employs 22,000 people, is currently rated 'excellent' by the Audit Commission.