Maitland has landed the bumper project brief for the transaction and will become Man's retained financial PR agency post-acquisition, replacing longtime incumbent Merlin PR.
Finsbury works as GLG's retained City agency and has been mandated for the deal.
GLG will become a wholly owned subsidiary of Man. Those involved with the transaction said it was too early to speculate whether Finsbury would continue to work for GLG in some capacity.
The cash and shares deal will create a hedge fund with a combined $63bn (£43bn) in assets under management. Both firms are based in the UK (although GLG is listed in the US), but are expected to use the tie-up to push aggressively into North America.
Maitland is understood to have begun working with Man in the weeks leading up to the announcement of the deal, having won a competitive pitch earlier this year. Maitland CEO Philip Gawith heads the account.
GLG Partners only brought in its first director of communications in November last year, when it hired Maitland's David Waller. Simon Anderson is global head of communications at Man Group.
Meanwhile, FD has been tasked with providing financial PR support to Jupiter Asset Management ahead of a London Stock Exchange IPO.
The British fund manager aims to raise around £220m, which would value it at around £1bn.
The FD team is led by FD MDs Andrew Walton, John Waples and Edward Gascoigne-Pees.
Alicia Wyllie, head of communications at Jupiter, said: ‘The team at FD impressed us with their knowledge of the sector and the issues. We have known Andrew and his team for a long time, but it has also been useful to have access to the expertise of former Sunday Times business editor John Waples to provide additional media insight.'
Jupiter, created in the mid-1980s and sold to Commerzbank, was bought out by management with the backing of the private equity group TA Associates in 2007 in a deal that FD also worked on.
In what proved to be a mini-wave of IPOs yesterday, Brazilian iron ore miner Ferrous Resources also announced its intention to float supported by Finsbury. The firm is expected to raise up to $400m on the London Stock Exchange to fund the development of its existing iron ore facilities and the construction of a port.