Noam Gottesman, co-founder of GLG, is taking a back-seat role at Man Group, the FTSE 100 firm that bought his Mayfair-based hedge fund business for $1.6bn (£1.04bn) just over a year ago.
Man is set to cut about 150 jobs, a tenth of its workforce, as it aims to reduce costs by $75m over the next year after the assets it manages fell by $6.1m, or 9.5 per cent, in the last three months of 2011.
Mr Gottesman, who is dating Charlie's Angels actress Lucy Liu and is worth $1.6bn according to Forbes magazine, is standing down as co-chief executive of GLG to become non-executive chairman of its US business.
It is understood he will devote his time to running his own wealth, continue his patronage of the arts and spend some time studying. He moved to New York two years ago.
He holds a 2 per cent direct stake in Man and, according to chief executive Peter Clark, has "substantial investments in GLG funds" both of which he has committed not to sell for another two years.
Man's funds under management fell from $64.5bn to $58.4bn between the end of September and the end of December. Some $2.5bn of this was due to outflows of funds and $1.5bn from market movements as its flagship AHL Diversified fund lost 7.7 per cent of its value in three months.