Johnson has been with Morningstar for nearly a decade, having joined from Mellon Global Investors as a senior investment consultant in January 2007.
He took on the role as head of portfolio management as part of a management restructure last year.
Johnson is moving to Cape Town, becoming head of investments at Nedgroup, in charge of its 'Best of Breed' range of investment solutions.
“After nearly 10 years I am departing Morningstar and heading for a new life in Cape Town,” he said.
“I am very proud of the team we have built for Morningstar’s investment management group in the EMEA region, and the capabilities and strategies we have developed in order to offer the highest quality asset allocation, manager selection and portfolio management services.
“The investment team will continue to be led by an experienced senior management team and furthermore, a number of additional hires are underway as expansion of our business has required.”
Dan Kemp, CIO, EMEA, continues to lead the London-based portfolio management team, with Diane Earnshaw and Ruli Viljoen as co-heads of manager selection.
The firm is planning to recruit two new people to the team, though these are likely to be less senior positions.
Kemp confirmed that Johnson will not be replaced directly, rather existing members of the team will be "developed into leadership positions".
He added: "This is a life decision for Robin. We are very sorry to lose him as he's a fantastic leader and a great investor, and a really valued member of the team."