Bagnall replaces Carroll in the MD post on 1 January as the group prepares to launch an office in London and recruit to boost the team.
Communique's London arm is due to be based in Burson-Marsteller's offices, following B-M's takeover of the Mancunian shop in 2001.
Bagnall, who joined Communique three years ago, said her promotion would allow Carroll to focus on strategy and expansion of the agency.
The agency previously employed joint MDs Brian Beech and Ian Leslie, but made Beech redundant in 2000, leaving Leslie as sole MD. Carroll took over the MD post following the agency's sale to B-M.
Bagnall has now created a six-strong management board, comprising group heads Justin Stevenson, Katrina Cooksey, Alex Shaida, Catherine Sandham and consultants Matt Hammerton and Kelly Harvarde.
'The idea is to share the responsibility and make people more accountable and responsible for client relationships,' said Bagnall on the creation of the board.
She added the agency's plans have B-M's full backing: 'Having had 18 months of B-M parentage, we are now looking to expand the offering. We want to get onto a different playing field and work for bigger brands.'
Communique currently employs 28 staff, but Bagnall said that number will expand to over 30 in the new year.
The agency is also looking internally and externally to hire a head for the London base.
Communique - which was founded by Carroll in 1986 - handles clients including Boddingtons and Manchester United Finance.