Marks served eight years on the Florida Public Service Commission (FPSC) and spent the last two as its chairman, after being appointed in 1979 by Governor Bob Graham. He served on the FPSC during the divestiture of AT&T and was instrumental in the implementation of the Federal Public Utility Regulatory Policies Act (PURPA). Before being appointed, he served as an Administrative Law Judge on the Commission. He is a partner in the Tallahassee -based Marks & Marks law firm. He has been an adjunct professor at FSU's College of Law, teaching utility regulatory law and a faculty member of the National Association of Regulatory Utility Commissioners'  utility rate school. Member of Omega Psi Phi Fraternity.
Marks' great-grandfather was the founder of the defunct historically black Coleman College in Gibsland, Louisiana. He and his wife, Jane, have a son, John Marks IV (born c. 1976).