Mitchell is currently OPG's chief nuclear officer and takes over the top post July 1.
He replaces Jim Hankinson, who is retiring after four years as OPG's president and chief executive.
OPG is a provincial Crown-owned utility company that generates and sells electricity.
Energy Minister George Smitherman said he was happy with the appointment, adding it recognizes the progress the company has been making.
Up until recently, OPG had been run by political appointees.
"I don't think it's about a move away from politics, but when you can hire an internal candidate it's recognition that the organization is functioning well and that there's a capacity to bring leaders up through the ranks," Smitherman said.
"This is an organization which is going through a lot of change."
The last internal OPG employee to be appointed as president was Al Holt in 1991.
Mitchell has been in charge nuclear since 2006, and was previously responsible for the operation of the four Pickering B nuclear reactors east of Toronto.