Andrew Formica, co-chief executive of Janus Henderson, is to collect $12m (£9.1m) in severance pay as he leaves the company.
Mr Formica, the chief executive of Henderson Global Investors before the firm merged with Janus, had been co-chief executive of the merged entity alongside Dick Weil, who came from Janus, since June last year.
But at a meeting on 31 July, the company said it has decided to place Mr Weil in sole charge, with Mr Formica exiting the business.
The company said the £9.1m severance cost included paying out part of the long-term incentive plan to which Mr Formica was entitled, ahead of schedule.
Mr Formica said: "It has been a pleasure to work with Dick in the creation and formation of Janus Henderson this past year.
"I am also proud of what we achieved at Henderson over the 10 years I was chief executive. Janus Henderson is an outstanding business with a fantastic and talented workforce. I wish Dick and the team the very best going forward."
Phil Wagstaff, global head of distribution for the business, is also leaving.
The information was contained in the quarterly results update for the company, covering the three months to 30 June.
The total assets under management for the company were £281bn across the combined business, with the company stating that positive investment performance outweighed the net outflows suffered by the business and negative currency movements.
There were net outflows of £2.05bn.
Adrian Lowcock, head of personal investing at Willis Owen, said the joint chief executive model is usually temporary, and the fact it has now changed is a positive sign for the prospects for the company.
He said having joint chief executives was a "useful tool" for a business to create the impression that there has been a merger of equals.