Burnside settled in Bristol, Rhode Island, where he became involved in the manufacture of firearms. In 1856 Burnside invented a highly successful breech-loading rifle.
On the outbreak of the American Civil War Burnside became a colonel in the Rhode Island Volunteers. After fighting successfully at Bull Run he was promoted to brigadier general in the Union Army. He served in North Carolina and developed a reputation as a dashing commander and during this period he is said to have popularized the fashion of side whiskers (later known as sideburns).
Burnside took part in the battle at Antietam (September, 1862) and afterwards President Abraham Lincoln asked him to replace George McClellan as commander of the Army of the Potomac. After the complaints that had been made by President Abraham Lincoln and the Secretary of War, Edwin M. Stanton, about the inaction of the Union Army, Burnside was determined to immediately launch an attack on the Confederate Army.
With a force of 122,000, Burnside, Joseph Hooker, Edwin Sumner, William Franklin attacked General Robert E. Lee and his army of 78,500, at Fredericksburg, Virginia, on 13th December. Sharpshooters based in the town initially delayed the Union Army from building a ponton bridge across the Rappahnnock River.
After clearing out the snipers the federal forces had the problem of mounting frontal assaults against troops commanded by James Longstreet. At the end of the day the Union Army had 12,700 men killed or wounded. The well protected Confederate Army suffered losses of 5,300. Ambrose Burnside wanted to renew the attack the following morning but was talked out of it by his commanders.
After the disastrous battle at Fredericksburg Burnside was replaced by Joseph Hooker. Burnside was put in charge of the Army of Ohio in March, 1863 and succeeded in capturing Morgan's Raiders and performed well at the siege of Knoxville.
Returning to the east he took part in the Wilderness campaign before organizing regiment of Pennsylvania coalminers to construct tunnels and place dynamite under the Confederate Army front lines at Petersburg. It was exploded on the 30th June and US Colored troops were sent forward to take control of the craters that had been formed. However, these troops were not given adequate support and the Confederate troops were soon able to recover its positions. Thousands of captured black soldiers were now murdered by angry Southerners.
After the war Burnside was successful in his engineering business and served as governor of Rhode Island (1866-69) and as a US Senator (1875-81). Ambrose Burnside died in Bristol, Rhode Island on 13th September, 1881.