Keller will go back to university next year to train as an orchestra conductor after more than a decade at asset management firm ICG.
Music has become a passion for the 52-year-old over the past few years, with conducting the overture to Mozart's opera The Marriage of Figaro a notable high point.
Keller said: "It has been a privilege to work at ICG and it is now the right time for me to further my musical studies and broader charitable interests.
"The Company is well-positioned to achieve its future strategic goals and I look forward to helping my colleagues deliver value for clients and shareholders in the coming months through the smooth transition to my successor."
Alongside his musical studies the chartered accountant intends to pursue charity work.
ICG chief executive Benoit Durteste said: "ICG is in excellent health and on track for another strong set of results.
"I have been grateful for Philip's wise counsel, particularly in the six years we have served together as senior management of the Company and I wish him well for his future outside executive life."
Prior to joining ICG in 2006 he was a finance director at global environmental consultancy Environmental Resources Charity.