De Mello, who ran the $626m (£385m) Dublin domiciled fund alongside Julia Ho and the rest of the Western Asset Asia team, is to leave the group on June 10.
Keith Gardner, the head of developing markets for Western Asset, will assume managerial responsibility of the fund. Ho, who has been at Western Asset since 2003, will partner Gardner in the macro positioning of the fund.
In a statement, Western Asset said: “Western Asset has always managed portfolios on a team basis. This means the loss of any single individual will not impact our management style or the implementation of our investment process.”