Hartman is retiring from the role but will remain in the UK-based hotel company’s board as a non-executive director.
Ren has been involved with the company since 1996, when he became a non-executive director.
He previously served as joint chief executive as well as interim chief executive prior to the appointment of Hartman in April 2008.
Ren was the chief financial officer and executive director since April 2001.
Millennium and Copthorne is now looking for a new chief financial officer and will also appoint a group chief operating officer.
Chairman Kwek Leng Beng said: "Wong Hong Ren emerged as the strongest candidate to fill the position after a prolonged search and many interviews conducted with other potential candidates."