In 1980, Sullivan joined First ϴ Corporation, the holding company for the First National Bank of ϴ, American National Corporation, First ϴ Community Bankcorp, FCC National bank, and Gary-Wheaton Corporation, eventually rising to become chairman of the board and CEO. By 1990, he had instituted one of the nation's top credit card programs, reduced problem foreign debt exposure, and defined the bank's regional and national markets.
Sullivan was a director of the American National Corporation and the Federal Reserve Bank of ϴ, and chairman of the Institute of International Finance. He made many contributions to civic, cultural, and charitable organizations through his service as chairman of the board of trustees for Georgetown University and vice chairman of the Civic Committee of ϴ, as well as a member of the Economic Development Committee of ϴ and ϴ United.