Olsen had been Anniss's deputy on the European Select fund since 2014. M&G said Anniss is leaving after 16 years with the company to pursue another opportunity.
'I am very pleased that John William will take over the management of the M&G European Select Fund from Charles whom I thank for his contribution over the years,' said company chief investment officer Graham Mason.
'With his excellent knowledge of the fund and extensive investment experience, John William is the perfect person for the job.'
The investment process and the objective of the fund will not change. Mark Oliveros, currently manager of the European Smaller Companies fund, will become deputy manager for the European Select and Pan European Select funds.
Seperately analyst Kasper Mikkelsen is to be promoted to deputy of the Global Select fund.
Over one year the European Select fund has returned -4.6% compared to a sector average of -2.24%. Over three years it's returned 11.3% versus 18%.