As a youth, Ellsworth longed for an appointment to West Point, but did not have the opportunity to prepare for its entrance exam. Leaving home in early youth, he lived in New York City, then Chicago, working as a law clerk and a solicitor of patents.
Ellsworth's interest to become a soldier led him to join a Chicago volunteer company about to disband for lack of interest. His enthusiasm and organizational skills revived the unit, attracting new members through his introduction of the Zouave dress and drill, patterned after French colonial troops in Algeria. His National Guard Cadets, later renamed the U.S. Zouave Cadets of Chicago, became highly efficient and disciplined, thanks to a regimen that stressed both tactics and the moral life. The zouaves gained attention throughout the Midwest by way of their baggy pants, short jackets, fezzes, and gaiters, as well as their intricate drill evolutions. Ellsworth was soon appointed a major on the staff of the commander of the Illinois National Guard, and his unit became the governor's guard. In 1860 his troops made a tour of major Northern cities, including New York and Washington D.C.
That August, Ellsworth went to Springfield, Ill., temporarily abandoning the company, to study law in the office of Abraham Lincoln. He remained, serving the future president in the fall campaign and accompanying him to Washington D.C., early in 1861. Ellsworth's subsequent attempt to secure a position in the War Department was hampered by the outbreak of the Civil War, leading him to New York City, to raise a regiment of volunteers. He recruited heavily among the city's fire departments, again clothing them in zouave uniforms and training them using the same regimen. The regiment was named the 11th New York Fire Zouaves, with Ellsworth its colonel.
On May 24, 1861, after returning to Washington, Ellsworth led his regiment across the Potomac and into Alexandria, Va., where he planned to occupy the town. Seeing a Confederate flag waving over the Marshall House hotel, Ellsworth climbed to the roof and tore it down. Moments later, while descending the stairs, he was shot to death by the proprietor, James T. Jackson, who was subsequently killed in a struggle with Pvt. Francis Brownell, a feat which would earn Brownell the Medal of Honor, in 1877.
Ellsworth, at 24, had become the North's first martyr, stirring its will to fight. Lincoln mourned openly over the loss, and his body lay in state at the White House before being returned to New York for burial.