O'Connor co-founded and has led the Irish firm for nine years, leading it to become one of the biggest emerging-market renewables developers in the industry.
"Eddie O'Connor has been the driving force behind Mainstream since its creation. In his new role, he will act as a thought-leader, an ambassador and advocate for both the company and for the renewables industry as a whole," new Mainstream chief Kinsella said.
Mainstream is O'Connor's second company — launched in 2008 — the first being developer Airtricity.
Since then he has led the company into new and developing markets, notably Chile and South Africa.
He was awarded the European Wind Energy Association's (now WindEurope) Poul La Cour award for outstanding achievements in the field of wind energy in 2014 and was made a Global Wind Energy Council ambassador in May 2016.
In the reshuffle, Andy Kinsella, who has been chief operating officer since July 2014, will become group chief executive, and current non-executive chairman, Sir Roy Gardner, becomes a senior independent non-executive director of the firm.
"Andy Kinsella has been managing Mainstream's operations for a number of years and has enormous experience of financing and constructing large power projects around the world," said O'Connor.
"I am very confident that Andy can lead the company to the next stage on its journey and capitalise on the enormous success we have had in recent years in developing wind and solar plant in the most promising, fast growing markets of the world," he added.
Mainstream has an 8.5GW development portfolio across wind and solar markets in Africa, Latin America and parts of Asia.