Remembered best as the driver of the vehicle in which United States President John F. Kennedy was riding at the time of his 1963 assassination, he was one of the Secret Service agents assigned to protect Kennedy.
After being employed for ten years as a servant of affluent New York and Boston families, he joined the United States Navy during the World War II years. He began his career with the U.S. Secret Service in the fall of 1945.
Because he did not immediately accelerate the car after shots were fired at Kennedy, Greer became a target of conspiracy theorists, some of whom believed that Secret Service agents may have plotted to kill the president.
A native of County Tyrone, Ireland, he relocated to the United States at the age of twenty and went on to raise a son named Richard.
He reportedly apologized to First Lady Jacqueline Kennedy for failing to prevent her husband's assassination.