It could be argued that King Richard I, Richard the Lionheart, was killed by a 'sniper' in 1199AD. Richard and his army had beseiged the Château de Chalus-Chabrol in France, and while walking in the open to view his men's work he was shot at by a crossbowman from the ramparts of the castle. While not immediately fatal, the wound became gangrenous, and he died a few days later. There's no information about whether the man who used the crossbow was a soldier by trade or a resident who had some training with a crossbow. Richard, as King, would have held the highest military rank within his armed forces.

And the book on my shelf where I read this is... my ancient copy of Gillingham's Richard the Lionheart from 1978.

The President of the United States of America (POTUS) is also the commander-in-chief of the U.S. Armed Forces.

So in that case, John F. Kennedy.