He was a lieutenant and adjutant with Company "E", 11th Regiment, Connecticut Infantry. He enlisted June 23 1862 and was killed at Cold Harbor, Virginia in 1864. During the Civil War, there were two battles fought at Cold Harbor, 10 miles northeast of Richmond, Virginia, the Confederate capital. The first battle (June 27, 1862) sometimes called the Battle of Gaines' Mill, was part of the Seven Days' Battles (June 25-July 1) which ended the Peninsular Campaign (April 4-July 1), the large-scale Union effort to take Richmond. The Second Battle of Cold Harbor (June 3-12, 1864) is considered one of the worst Northern defeats of the Civil War. It was there that Samuel Barnum received the wound that resulted in his death. Following the Battle of Spotsylvania Court House (May 8-19), Union General Ulysses S. Grant advanced southward toward Richmond in a series of flanking movements. Confederate troops under General Robert E. Lee at Cold Harbor entrenched themselves in defensive positions behind earthworks. From these, Union assaults were repulsed, with heavy losses to the attackers. Because of Grant's vast numbers (more than 100,000 men) his losses of about 7,000 (compared with fewer than 1,500 for Lee) did not deter him from continuing to Petersburg later that month in his drive toward Richmond.
Samuel was wounded at Cold Harbor, Virginia on June 3, 1864, and died at Washington, DC, on June 19, 1864, aged 26 years.