The head of one of the New York City region's biggest power companies resigned Tuesday evening amid a turbulent period for the utility, which has been under fire for its performance after Sandy wiped out power to almost 90% of its 1.1 million customers.

The acting chief executive of Long Island Power Authority, Michael Hervey, said in an interview he had planned for months to step down. His decision Tuesday came on the day that New York Gov. Andrew Cuomo said he would form a special state commission...