Casey entered West Point in 1915, where his best friend and roommate was Lucius D. Clay. At West Point, Casey played football as a halfback, substituting for Elmer Oliphant. One of Casey's duties was tutoring Oliphant in mathematics. Casey decided that winning games was more important than playing, and he helped keep Oliphant proficient at math. Unlike most appointees to West Point, a grateful Casey wrote frequently to Griffin about his progress and sent him football tickets. When Casey's younger brother Martin Charles Casey wanted to enter West Point, Griffin directly appointed him to the class of 1920 without having to pass the examination. Martin served with the coastal artillery for eleven years before being medically discharged due to migraine headaches on 30 November 1931. Martin later became a successful lawyer. Both brothers acquired the nickname "Pat" at West Point.