Cullen was a beloved teacher, and many of his students became influential figures. He kept in contact with many of his students, who included; Joseph Black; Benjamin Rush, a central figure in the founding of the United States of America; John Morgan, who founded the first medical school in the American colonies, the Medical School at the College of Philadelphia); William Withering, the discoverer of digitalis; Sir Gilbert Blane, medical reformer of the Royal Navy; and John Coakley Lettsom, the philanthropist and founder of the Medical Society of London.