Philip Willcock will leave RPMI Railpen where he has been CEO since February, said a statement from the firm.
RPMI manages the assets of the £28 billion ($35.6 billion) Railways Pension Scheme, London.
Babloo Ramamurthy, chairman of RPMI, will support the trustee of the Railways Pension Scheme until a new CEO is chosen. A spokesman for RPMI did not respond to a request for comment on the appointment process.
"We will be seeking to recruit a high-caliber successor who will help deliver RPMI's mission of paying member's pensions securely, affordably and sustainably and provide support to the trustee" of the pension fund, said Mr. Ramamurthy in the statement.
Mr. Willcock was appointed CEO in February following the departure of Chris Hitchen.
Mr. Willcock will join AIG Life (U.K.), a division of American International Group, as CEO, said a news release from American International Group. The appointment is subject to regulatory approval and will be effective March 1.