In 1963, McCullough moved to the United Kingdom for four years. At Great Ormond Street Hospital in London, she met the chairman of the neurology department at Yale University. He offered her a research associate job at Yale. McCullough spent ten years from April 1967 to 1976 researching and teaching in the Department of Neurology at the Yale Medical School in New Haven, Connecticut, United States. It was while at Yale that she wrote her first two books.