Inspired and influenced by painters such as Neil Welliver, Fairfield Porter and Jane Freilicher, Sullivan was one of many artists reviving the art of landscape painting in the late 1960s. At first he painted people in vague landscapes but then started to develop the landscape itself. He became fascinated with the Hudson River, setting up his easel on the old, raised West Side Highway. He studied the work of the 19th-century Hudson River school artists and later traveled through Colombia, painting many of the scenes depicted by Hudson River School painter Frederic Church. This experience transformed his sense of coloration and led him to focus on spiritual, not merely documentary, aspects of landscape. Living near Times Square for many years, Sullivan frequently painted the river and eventually settled further north in Hudson New York, near Olana, the home of Frederic Church.