Greenbrook will provide corporate and financial advice to both Electra Private Equity and Electra Partners, the independent private equity fund manager that manages the listed entity.
The former M: client has switched to Greenbrook following a competitive pitch.
Andrew Honnor, managing partner of Greenbrook, said he was "delighted to be advising Electra, one of the oldest and most respected firms in the private equity industry".
Alex Cooper-Evans, investment partner and head of investor relations at Electra Partners, hailed Greenbrook’s "specialist knowledge of the sector" and added: "We are confident we will build a very close relationship with them."
Electra is one of a swathe of clients and staffers switching to new agencies following the news that M:Communications was to wind down at the end of the summer.
Other M: clients to have found a new home in recent weeks include FTSE 100 miner ENRC shifting to Capital MSL and 888 Holdings moving to Hudson Sandler.