John Fulton Reynolds was born in Lancaster, Pennsylvania on September 21*, 1820, the son of John and Lydia Moore Reynolds. He graduated from West Point in 1841, ranking 26th in a class of 52. His career in the Regular Army included the usual succession of assignments to garrison duty, punctuated by service in the Mexican War, the Utah Expedition, and various campaigns against the Indians. He received two brevet promotions for gallantry and meritorious conduct during the Mexican War. In 1860 he was appointed commandant of cadets and instructor of tactics at West Point, where he served until the outbreak of the war.

On May 14, 1861, Reynolds was promoted and assigned as lieutenant colonel of the newly activated 14th U.S. Infantry. He was promoted to brigadier general of volunteers on August 26th. He commanded troops in the defense of Washington, served as military governor of Fredericksburg, then took command of a brigade in the Army of the Potomac in June of 1862. Reynolds was captured by the Confederates after falling asleep and being cut off from his troops after the battle of Gaines' Mill (June 28, 1862), and was exchanged in August. He commanded a division of the Pennsylvania Reserves at Second Manassas and led the Pennsylvania militia during the Maryland campaign.

Promoted to major general, U.S. Volunteers on November 29, 1862, Reynolds commanded the First Corps of the Army of the Potomac at Fredericksburg and Chancellorsville. Before being relieved of command of the Army of the Potomac three days before Gettysburg, Joe Hooker placed Reynolds in command of the three corps which formed the left wing of the army. When Buford's cavalry encountered significant Confederate forces at Gettysburg on July 1, 1863, Reynolds immediately rushed his First Corps into action and ordered up the other two corps under his command. While personally directing the deployment of the famous Iron Brigade, he was struck down by a rifle bullet, dying almost instantly.

John Fulton Reynolds was the epitome of the thoroughly competent, completely reliable, hard-fighting officer of the "Old Army." Possessed of considerable personal courage and an excellent tactical sense, he was trusted and respected by both his subordinates and his superiors. Joe Hooker called him "the ablest officer" under his command. Perhaps the highest compliment paid him was the nickname bestowed on him by his troops: "Old Common Sense." On the first day of the battle of Gettysburg, the Union Army lost one of its most solidly capable general officers.