Apollo said Harris will leave his role when the $11 billion all-stock merger with annuities provider Athene Holding Ltd closes in the first quarter of next year. Harris, 56, will retain his seat on Apollo's board and executive committee, the firm said.

The move comes after Apollo co-founder Leon Black stepped down from all his executive roles at the private-equity firm earlier this year, in the wake of a law firm report that revealed he had paid late financier and convicted sex offender Jeffrey Epstein $158 million for advice on tax and estate planning and related services between 2012 and 2017. The review had cleared Black of any wrongdoing.

In March, Black, who co-founded Apollo 31 years ago alongside Harris and Marc Rowan, stepped down as the company's chairman, with Jay Clayton, former U.S. Securities and Exchange Commission chief, taking over as non-executive chairman. Earlier in January, Black relinquished his post as chief executive officer to Rowan after the report was published.

In elevating Rowan to become CEO, Apollo passed over Harris, who had been widely expected to take over the top job from Black.

Margaret Brown, a member of the board of the California Public Employees' Retirement System (CalPERS), said she will take a wait-and-see approach on how Rowan fares as CEO.

"What I'm looking for is stability, honesty and ethics in that leadership," Brown said.

Analysts on Thursday said Harris' departure from his day-to-day responsibilities was expected given Rowan's promotion and will not impact Apollo's fundraising of its flagship funds.

"The announcement seems orderly and, based on our conversations with investors, inevitable given the recent promotion of Mr. Marc Rowan to the CEO role," Citi analysts wrote in a note. "Beyond the headline nature of the update, we do not think it will have an impact on PE Fund IX or the upcoming Fund X capital raise."

Apollo said Harris will turn his attention to his investing business, his family's foundation and HBSE, a sports and entertainment company he co-founded with David Blitzer, a Blackstone Group Inc senior managing director. HBSE's brands include National Hockey League's New Jersey Devils and the National Basketball Association's Philadelphia 76ers.

"After nearly 31 years at Apollo, it is time for me to start the next chapter of my career, where I will focus full-time on the platforms I've created outside of the firm," Harris said in a statement on Thursday.

Apollo's shares were trading at $56.48, down 0.89% as of 3PM ET on Thursday.

(Reporting by Niket Nishant and Noor Zainab Hussain in Bengaluru and Chibuike Oguh and Jessica DiNapoli in New York;Editing by Vinay Dwivedi and Cynthia Osterman)

By Chibuike Oguh and Jessica DiNapoli