[3] Reynolds was nominated to the United States Military Academy in 1837 by Senator James Buchanan, a family friend, and graduated 26th of 50 cadets in the class of 1841.

From 1842 to 1845 he was assigned to St. Augustine, Florida, and Fort Moultrie, South Carolina, before joining Zachary Taylor's army at Corpus Christi, Texas, for the Mexican–American War.

He was awarded two brevet promotions in Mexico – to captain for gallantry at Monterrey and to major for Buena Vista, where his section of guns prevented the Mexican cavalry from outflanking the American left.

He was appointed lieutenant colonel of the 14th U.S. Infantry, but before he could engage with that unit, he was promoted to brigadier general on August 20, 1861, and ordered to report to Washington, D.C.

The brigade was hit hard by the Confederate attack of June 26 at the Battle of Beaver Dam Creek, but their defensive line held and Reynolds later received a letter of commendation from his division commander, Brig.

[7] The Confederate attack continued on June 27 and Reynolds, exhausted from the Battle of Gaines' Mill and two days without sleep, was captured in Boatswain's Swamp, Virginia.

His counterattack halted the Confederate advance long enough to give the Union Army time to retreat in a more orderly fashion, arguably the most important factor in preventing its complete destruction.

Hooker originally placed the I Corps on the extreme left of the Union line, southeast of Fredericksburg, hoping to threaten and distract the Confederate right.

Reynolds, who had gone to sleep after giving his proxy vote to Meade, woke up and muttered loud enough for Hooker to hear, "What was the use of calling us together at this time of night when he intended to retreat anyhow?"

President Abraham Lincoln met with Reynolds in a private interview on June 2 and is believed to have asked him whether he would consider being the next commander of the Army of the Potomac.

Reynolds supposedly replied that he would be willing to accept only if he were given a free hand and could be isolated from the political influences that had affected the Army commanders throughout the war.

[13] On the morning of July 1, 1863, Reynolds was commanding the "left wing" of the Army of the Potomac, with operational control over the I, III, and XI Corps, and Brig.

For the Union side, the death of John Reynolds meant more than the loss of an inspiring leader; it also removed from the equation the one person with enough vision and sense of purpose to manage this battle.

In the command confusion that followed Reynolds' death, the two Union corps that reached the field were overwhelmed and forced to retreat through the streets of Gettysburg to the high ground south of town, where they were rallied by his old friend, Maj. Gen. Winfield S.

A letter from Reynolds' sister, Jennie, stated that the wound had a downward trajectory from the neck, implying that he was shot from above, presumably a sharpshooter in a tree or barn.

Historians Bruce Catton and Glenn Tucker make firm assertions that a sharpshooter was responsible; Stephen Sears credits volley fire from the 7th Tennessee against the 2nd Wisconsin; Edwin Coddington cites the sister's letter and finds the sharpshooter theory to be partly credible, but leans towards Sears' conclusion; Harry W. Pfanz agrees that the location was behind the 2nd Wisconsin, but makes no judgment about the source of the fire.

The film portrays Reynolds as being deliberately targeted by a Confederate sharpshooter, a scene based on the Don Troiani painting of the event.

Reynolds is also significant in the prequel to The Killer Angels, Jeffrey Shaara's novel Gods and Generals, although his role was deleted from the 2003 film based on the novel.