In 1906 Kennedy received the Bachelor of Medicine degree from Queen's University in Belfast and in 1911 the Doctor of Medicine degree from the Royal Irish University in Dublin. Between these years he received his neurological training in London at the Hospital for the Paralyzed and Epileptic (now the National Hospital, Queen Square), one of the major centers of clinical neurology. As a house officer at the hospital Kennedy was influenced by William Gowers, Hughlings Jackson, Henry Head, Kinnier Wilson, and Victor Horsley, one of the first neurosurgeons.
In 1910 Kennedy immigrated to the United States, where, at the age of twenty-six, he became chief of clinic at the newly founded Neurological Institute of New York. As attending and, later, consulting neurologist he was active in that institution until 1934. In 1915 Kennedy became chief of the vast neurological division of Bellevue Hospital, New York City's largest municipal hospital. He held this post for thirty-four years.
During World War I Kennedy, who rose to the rank of major, served with the British and French armies and directed a French military hospital. Far from interrupting his career as a neurologist, the frontline experience--involving several brushes with death--increased his competence in dealing with both the physical and mental effects of shellfire on the nervous system. His interpretation of shell shock has remained valid as the conflict between individual and group claims: self-preservation against endurance. Returning to New York after the war, Kennedy resumed his professional work.
Kennedy became a naturalized United States citizen in 1938. He died on his own teaching ward in Bellevue Hospital. He is well known to neurological specialists by the syndrome that bears his name. It consists of loss of vision in one eye as a result of the compression of its optic nerve, which is rendered atrophic by an adjacent tumor. The other optic nerve shows papilledema because of the generalized swelling of the brain. Kennedy published his observations of the syndrome in 1911; and although two years earlier William Gowers and Leslie Paton had independently made similar interpretations, Kennedy received acclaim for his lucid and comprehensive description of the phenomenon.
Kennedy admired Freud, but he rejected the Freudian notion of sexual repression as the dominant force in the dynamic of inner conflict. He opposed those he saw as "gadgeteers of science, " who he felt followed a mindless technology that was undermining the exercise of critical clinical judgment.
Kennedy was a fellow of the Royal Societies of Medicine of Edinburgh and of London, an honorary member of many foreign medical organizations, and president of the American Neurological Association (his timely presidential address of 1940 dealt with the role of the scientist faced with world catastrophe). He also belonged to several fashionable societies. As a member of the Charaka Club--so named after the founder of Hindu medicine--he joined a group of doctors concerned with history, literature, and art in medicine.
Kennedy had an impressive personality as well as profound neurological skills. Tall, handsome and of a gregarious disposition he had, from adolescence, theatrical aspirations and interests which he never lost. He was charismatic and completely at ease in the USA. He was a major figure in New York society. He was on cordial terms with actors, writers, musicians, and politicians; and his aspiring young medical friends mingled at his dinner parties with many celebrities. Not uncommonly, following an evening out, the professor, wearing an opera cloak and Edwardian monocle and accompanied by one or more of his medical guests, would make a reappearance at the hospital to answer an emergency call. Frequently he would slip a few dollars into the hand of a penurious patient while examining him.
In 1912 Kennedy married Isabel Stevenson McCann of Belfast; they had one daughter before the marriage ended in divorce. Around 1943 Kennedy married Katherine Caragol de la Terga; they had one daughter.