Clint Hill, former US Secret Service agent who was in the presidential motorcade during the assassination of John F. Kennedy. He is the last surviving passenger of the presidential limousine

Clinton J. Hill (born 1932) is a former United States Secret Service agent who was in the presidential motorcade during the assassination of John F. Kennedy. After Kennedy was shot, Hill ran from the car immediately behind the presidential limousine and leapt onto the back of it, holding on while the car raced to Parkland Memorial Hospital. This action was documented in the famous Zapruder film. Hill is the last surviving passenger of the presidential limousine which arrived at Parkland.
Hill, a native of Washburn, ND, attended Concordia College (Moorhead) in Moorhead, MN where he played football, studied history, and was a 1954 graduate. After college he was assigned to the Denver office of Secret Service in 1958. After John F. Kennedy was elected President of the United States, Hill was assigned to protect the First Lady, Jacqueline Kennedy. Hill became a nationally-known figure upon the assassination of President Kennedy on November 22, 1963.
Hill remained assigned to Mrs. Kennedy and the children until after the 1964 presidential election. He then was assigned to President Lyndon B. Johnson at the White House. In 1967, when Johnson was still in office, he became the Special Agent in Charge (SAIC) of Presidential protection. When Richard Nixon came into office, he moved over to SAIC of protection of Vice President Spiro Agnew. Finally, Hill was assigned to headquarters as the Assistant Director of the Secret Service for all protection. He retired in 1975.

President Kennedy was assassinated in Dallas, Texas, during a motorcade through the city while en route to a luncheon at the Dallas Trade Mart. The President and Mrs. Kennedy were riding in an open limousine containing three rows of seats. The Kennedys were in the rear seat of the car, and the Governor of Texas, John Connally, and his wife, Nellie Connally, were in the middle row. Secret Service agent William Greer was driving and the president's bodyguard, Roy Kellerman, was also in the front seat.

Hill was riding in the car that was immediately behind the presidential limousine. As soon as the shooting began, Hill jumped out and began running to overtake the moving car in front of him with the plan to climb on from the rear bumper and crawl over the trunk to the back seat where the President and First Lady were located.
Hill grabbed a small handrail on the left rear of the trunk that was normally used by bodyguards to stabilize themselves while standing on small platforms on the rear bumper. According to the Warren Commission's findings there were no bodyguards stationed on the bumper that day because

...the President had frequently stated that he did not want agents to ride on these steps during a motorcade except when necessary. He had repeated this wish only a few days before, during his visit to Tampa, FL. .

The notion that the President's instructions in Tampa jeopardized his security in Dallas has since been denied by Hill and other agents. Regardless of Kennedy's statement photos taken of the motorcade along earlier segments of the route show Hill riding on the step at the back of the car.
As an alternate explanation fellow agent Gerald Blaine cites the location of the shooting:

We were going into a freeway, and that's where you take the speeds up to 60 and 70 miles an hour. So we would not have had any agents there anyway.

Hill grabbed the handrail less than two seconds after the fatal shot to the President. The driver then accelerated, causing the car to slip away from Hill, who was in the midst of trying to leap on to it. He succeeded in regaining his footing and jumped on to the back of the quickly accelerating vehicle.

As he got on, he saw Mrs. Kennedy, apparently in shock, crawling onto the flat rear trunk of the moving limousine (he later told the Warren Commission that he thought Mrs. Kennedy was reaching for a piece of the President's skull which had been blown off). Agent Hill crawled to her and guided the First Lady back into her seat. Once back in the car, Hill placed his body above the President and Mrs. Kennedy. Meanwhile, in the folding jump seats directly in front of them, Mrs. Connally had pulled her wounded husband, Governor John Connally, to a prone position on her lap.
Agent Kellerman, in the front seat of the car, gave orders over the car's two-way radio to the lead vehicle in the procession "To the nearest hospital, quick!" Hill was shouting as loudly as he could "To the hospital, to the hospital!" Enroute to the hospital, Hill flashed a "thumbs-down" signal and shook his head from side to side at the agents in the followup car, signaling the graveness of the President's condition.