He was born in New York City to a wealthy family, and early in life desired a career in the military. His parents died when he was young, and he was consequently raised by his grandfather, who insisted against the younger Kearny's wishes that he pursue a law career. Kearny attended Columbia College, attaining a law degree in 1833.
In 1836 his grandfather died, leaving Kearny a fortune of over $1 million. The following year Kearny obtained a commission as a second lieutenant of cavalry, assigned to the 1st Dragoons, who were commanded by his uncle, Colonel Stephen W. Kearny, and whose adjutant general was Jefferson Davis. The regiment was assigned to the western frontier.
Kearny was sent to France in 1839 to study cavalry tactics, first attending school at Saumur and then participating in several combat engagements with the Chasseurs d'Afrique in Algiers. He returned to the United States in the fall of 1840 and prepared a cavalry manual for the Army based on his experiences overseas.
Shortly afterward, he was designated aide-de-camp to General Alexander Macomb, and continued to serve in this position until Macomb's death in June of 1841. After a few months at the cavalry barracks in Carlisle, Pennsylvania Kearny was assigned to the staff of General Winfield Scott, soon becoming his aide-de-camp. He did additional duty on the frontier, accompanying his uncle's unit on an expedition to the South Pass of the Oregon Trail in 1845.
Kearny, disappointed with the lack of fighting he was seeing in the Army, resigned his commission in 1846, but returned to duty only a month later at of the outbreak of the Mexican American War. He raised a company of cavalry for the conflict and, notably, equipped the troops lavishly with funds from his personal fortune. This unit was originally stationed at the Rio Grande but soon became the personal bodyguard for General Scott, the commander in chief of the Army in Mexico. Kearny was promoted to captain in December of 1846.
Kearny and his men participated in the battles of Contreras and Churubusco; in the latter engagement Kearny suffered a gunshot to his left arm, which later had to be amputated. He nevertheless quickly returned to duty, and when the U.S. Army entered Mexico City the following month, Kearny had the personal distinction of being the first man through the gates of the city.
After the war Kearny did a stint with the Army recruiting service in New York City. While there he was presented with a sword by the Union Club for his service during the war, and was promoted to major.
In 1851 he was a member of a unit which saw action against the Rogue River Native American tribe in California. After the failure of his marriage, frustrated with the slow promotion process of the Army, Kearny resigned his commission in October of that year, and embarked on a trip around the world, visiting China, Ceylon, and France before returning home to semi-retirement at his estate, "Belle Grove".
1859 saw Kearny returning to France, re-joining the Chasseurs d'Afrique who were at that time fighting against Austrian forces in Italy. Later he was with Napoleon III's imperial guard at the Battle of Solferino, where cavalry under his command played a key role in the victory there. For this action, he was awarded the French Légion d'honneur, becoming the first U.S. citizen to be thus honored.
When the American Civil War broke out in 1861, Kearny returned to the United States and was appointed a brigadier general, commanding the 1st New Jersey Brigade, which he trained efficiently. His brigade, even after he left to command a division, performed spectacularly, especially at the Battle of Glendale. He led them into action at the Battle of Williamsburg and the Battle of Fair Oaks; after the latter he was promoted to major general. At Williamsburg, leading his brigade onto the field, Kearny said, "I'm a one-armed Jersey son-of-a-gun, follow me!" His performance during the Peninsular Campaign earned him much respect from the army and his superiors. However, he held much contempt for the commander of the Army of the Potomac, Major General George B. McClellan, whose orders (especially those to retreat) he frequently ignored.
Kearny is credited with devising the first unit insignia patches used in the U.S. Army. In the summer of 1862, he issued an order that his officers should wear a patch of red cloth on the front of their caps to identify themselves as members of his unit. The enlisted men, with whom Kearny was quite popular, quickly followed suit of their own volition. Members of other units picked up on the idea, devising their own insignia, and these evolved over the years into the modern shoulder patch.
In the following months, Kearny was engaged at the Second Battle of Bull Run and the indecisive Battle of Chantilly, as well as dozens of minor skirmishes, all with more or less favorable results. Unfortunately, while performing reconnaissance near the Confederate lines, he was shot in the back and died immediately. When Confederate General A.P. Hill, an acquaintance, noticed Kearny's body, he stated, "Poor Kearny! He deserved a better death than that." His body was returned to the Union, accompanied with a note by General Robert E. Lee. He was buried at Trinity Churchyard in New York. In 1912 the remains were exhumed and re-interred at Arlington National Cemetery.
The city of Kearny, New Jersey is named in his honor. Likewise, a fort in Wyoming was named for Kearny; it had but two years of existence. New Jersey is represented by a statue of Kearny in Statuary Hall in the U.S. Capitol. See references.