Konarzewski has left the agency and is taking a short break before commencing her new role on 1 January.
Man Group has 1,200 employees worldwide with $52.5 billion of assets under management.
She is replacing David Waller, who joined the Association for Financial Markets in Europe as director of external relations in November. Her responsibilities will include internal and external communications as well as the firm's CSR initiatives and sponsorships, including the Man Booker prize for literature.
She has been compensated for her small equity stake in Greenbrook, which she joined as a partner at launch in February 2012.
Greenbrook has recently bolstered its senior team with the hiring of Charlotte Balbirnie from the now defunct M: Communications and Georgiana Brunner, previously communications manager at private equity company Apax Partners.
Andrew Honnor, founder of Greenbrook, said: "Rosanna is an extremely talented individual, a great friend of the company and we are sad to see her go. We wish her the very best in what is a fantastic opportunity."
Rosanna Konarzewski leaves Greenbrook for Man Group global role
Rosanna Konarzewski, one of the founding partners of financial PR agency Greenbrook, is moving to join asset management company Man Group as global head of communications.
