US/UK - CalPERS and The Pensions Regulator (TPR) have lost senior staff.
Johnson said: "This new position is the next step on my career ladder. It presents a tremendous opportunity to apply my passion in a private sector environment and an opportunity to work for a company that shares the vision for high standards of corporate governance."
CalPERS said Johnson would also step down from his chairmanship of the Council of Institutional Investors Board of Directors.
Until a replacement is found, Eric Baggesen, who joined at the end of May as senior investment officer, public equities, has assumed the role.
Johnson's departure is the latest in a series of senior exits from CalPERS this year including CIO Russell Read, CEO Fred Buenrostro and SIO Christianna Wood.
Martine Trouard-Riolle, head of corporate risk management, TPR, has lost her position as a result of a management restructure after three years at the body.
June Mulroy, business delivery director, credited Trouard-Riolle with enhancing the clearance process and successfully leading a number of anti-avoidance cases as a critical part of the organisation's risk-based approach to regulation.
Trouard-Riolle's responsibilities have been taken over by Ian Cordwell until a new management structure has been established.
Most respondents in this week's Pensions Buzz do not think businesses should be able suspend AE contributions if in financial distress.
Former BHS owner Dominic Chappell has lost the appeal against his section 72 conviction and sentence for failing to hand over information to The Pensions Regulator (TPR).
This week's top stories include Marsh and McLennan Companies agreeing to buy JLT, and the home secretary calling for AE to be scrapped in a no-deal Brexit scenario.
Lesley Titcomb says the watchdog wants closer interactions with pension funds to spot problems sooner and act before having to use its more stringent powers