Mainwaring was named in the role earlier this month, but the company could not complete the appointment until it secured the relevant approval.
Mainwaring was previously a director of Tullett Prebon from 2006 to 2016.
Mainwaring will replace Mark Ward, who is to exit the company after a short handover, taking the role on a permanent basis and also joining the company’s board.
Andy Green, chairman of IG Group, said: “After an extremely thorough search process, we are very pleased to have secured someone of Paul's standing."
“His knowledge of the financial services sector and experience in several public companies means he can hit the ground running and immediately add to the operational and strategic progress at IG,” he said.