While Mr Williams will remain with Platypus AM in a "key business advisory and mentoring role", his role as chief executive officer will be assumed by current chief operating officer Gary Adamson.
Platypus co-founder Nick Wright has been appointed chairman of the boutique fund manager, with Rohan Mead deputy chair.
"We are extremely grateful for Donald’s vision and leadership over the past 10 years. Along with Gary and Nick, he has been instrumental in building one of the longest standing and strongest investment boutiques in the market," Mr Mead said.
Mr Wright said Mr Williams will be "sadly missed".
"As a result of Donald’s efforts to build a strong team and robust investment process we are very pleased to have individuals of the calibre of Prasad Patkar and Peter Brooke, Platypus veterans, able to step into the head of investment roles," Mr Wright said.
Prasad Patkar will assume the role of head of qualitative investments, with responsibility for the management of the flagship investment portfolios and oversight of the fundamental investment team and processes.
Peter Brooke, portfolio manager for Platypus’ systematic portfolios, has been appointed head of quantitative investments.
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