Born in 1833, Spencer, who had little formal education, began working in a silk mill at the age of fourteen, where he demonstrated a particular genius for designing machinery. His lifelong interest in firearms led to employment with Colt Firearms and then Robbins and Lawrence, contractor for the famed Sharps. Spencer also worked on his own ideas and, on March 6, 1860, was awarded a patent on a new type of breechloading repeating rifle.

The introduction of the self contained metallic cartridge in the 1850s made the Spencer Model 1860 technically possible. The Spencer, with its tubular buttstock magazine, was a simpler and sturdier design than the competing Henry. Although the gun's seven round capacity was less than half that of the Henry, the Spencer's .56-.56 rimfire cartridge packed a much heavier wallop than the .44 rimfire. A lever action gun like its competitor, the Spencer's lever actuated a rolling block that fed cartridges from the magaznie into the chamber. Unlike the Henry, however, the Spencer's hammer had to be manually cocked for each shot.

The advent of Civil War promised a big market to Spencer as well as a legion of other firearms inventors. In the summer of 1861, the inventor secured manufacturing space in the Chickering Piano Factory in Boston, and pressed the government to test his prototypes.

Like almost all "patent arms" purveyors, Spencer used any influence available to promote his repeater. Fortunately for him, a former neighbor who was a personal friend of Navy Secretary Gideon Welles secured a trial for the new gun. Although the Henry was also tested and both weapons received favorable reports, the Navy ordered ordered 700 Spencer rifles with 30 inch barrels and sword bayonets.

The Army also tested the Spencer and preferred it to the Henry. Ordnance people liked the Spencer's sturdiness, heavy caliber and relative ease of manufacture and placed an order for 10,000 (later reduced to 7,500) rifles. Although the army wanted arms in the standard .58 caliber bore diameter, Spencer delivered his guns in .56-.56, which was nominally .52 caliber. Actually, there is a great deal of variation in .56-.56 bore diameters, many of which are actually .54 caliber. In black powder days, bore tolerances were not considered as critical as today. The charcoal based propellant "boosted" undersized bullets up to bore diameter, atoning for a multitude of machining sins.

Deliveries of Spencer rifles began in December of 1862 and continued through June of 1863. The state of Massachusetts also ordered 2,000 rifles, although other contracts delayed delivery of the Bay State guns until 1864. A total of 11,471 Spencer rifles were manufactured, and it is safe to say most made their way to the battle front during the war.

More numerous were the carbines, with orders beginning in the summer of 1863. Spencer's salesmanship was fully equal to his skills as an inventor and manufacturer, and he toured battlefronts and visited the White House promoting his rifle. After a personal test by President Lincoln, the Spencer's future was secure.

Carbine deliveries began in October of 1863, and a total of 64,685 were delivered through January 1866. In addition the government purchased 30,496 of the slightly different pattern .56-.50 caliber Model 1865 Spencer carbines, made by the Burnside Rifle Company of Providence, Rhode Island.

Some writers have incorrectly assumed that all these guns saw service in the Civil War, which gives a skewed view of the Spencer's influence. Including deliveries of 2,007 carbines on April 3 and April 12, 1865, as hostilities came to a close, 46,185 carbines were deliverd by April 12, 1865. Adding the 11,471 rifles gives a total of 57,656 Spencers which possibly saw combat. All of the Burnside guns were delivered after the close of hostilities.

The first Spencer was fired at Confederates in a skirmish near Cumberland, Maryland on October 16, 1863. The prototype gun was in the hands of Sergeant Francis O. Lombard, a gunsmith friend of the inventor serving in Company F, 1st Massachusetts Cavalry. It was a while before others would follow Lombard's example. Most, if not all of the rifles puchased by the Navy were issued to the Mississippi Marine Brigade. In January of 1863, the army issued its first Spencers to the 5th, 6th 7th and 8th Independent Companies of Ohio Sharpshooters.

Spencers gained their greatest fame in the hands of men who didn't wait for the army to issue them. Colonel John T. Wilder's Mounted Infantry Brigade bought their own repeaters. Wilder originally wanted Henrys for his men, but slow production severely limited the availability of that arm. Eventually Wilder's regiments, including the 17th, 72nd, 92nd Indiana, and 98th Illinois (the 92nd Illinois joined the brigade later) were armed with a mixture of Spencer rifles and carbines along with Burnside single shot breech loading carbines.

On June 24, 1863, Wilder's men cleared Hoover's Gap, Tennessee of Confederates and then held it successfully against a counterattack. Although often cited as a prime example of the efficacy of the Spencer's firepower, the fight at Hoover's Gap is as much a tribute to Wilder's speed of movement and tactical abilities as his men's armament. His brigade overran the single Rebel regiment in the gap and then, supported by an artillery battery, stood off a counterattack by a weak brigade of Confederates. Caught in flank and front by Spencer fire and cannister, 650 Grayback attackers lost 19 killed and 126 wounded, a fairly heavy percentage, but hardly a massacre. Chickamauga provided a better test of the effects of rapid fire. There is no doubt that Wilder's fast firing Spencers caused General Longstreet to believe he was confronting a whole army corps, and helped to save the defeated Union Army.

Spencer Rifles were first carried into battle by the 5th Michigan Cavalry at Hanover Station and were later used at Gettysburg in the drawn cavalry battle behind the Union lines on July 3, 1863. General Custer credited the Spencers of the dismounted 5th with enabling the regiment to hold a crucial fenceline against the Rebels. In fact, the 5th was never directly attacked, although its rapid flanking fire wrecked one of several Rebel charges. Mounted saber charges by other Union horsemen, the superior accuracy and ammunition supply of the Federal horse artillery and the fact that some of Stuart's men had to withdraw when they ran out of ammunition were also factors in the fight, however.

One historian credits the handful of Spencers issued to Colonel William Gamble's cavalry brigade for the stand taken by the Federal cavalry on Gettysburg's first day. This seems doubtful, however. Despite the claims of participants, the Yankee cavalry does not seem to have been heavily engaged with Confederate infantry on McPherson's Ridge. With 1,600 troopers engaged, Gamble only lost 13 men killed and 58 wounded.