Union General John Fulton Reynolds of Lancaster, PA, was killed in the opening hours of the Battle of Gettysburg. If memory serves me correctly, he was also the highest ranking officer killed during the battle.
John Fulton Reynolds - Civil War Union Major General. One of the top Union Generals in the Civil War. Rose from commanding a brigade to commander for the Army of the Potomac's I Corps. Fought in Battles of Second Bull Run, Fredericksburg, Chancellorsville and Gettysburg. While posting elements of the famed "Iron Brigade" at Gettysburg he was shot and instantly killed in a Confederate volley, thus making him both the first and the highest ranking General to die in the engagement.