William Nelson began his career in the navy after receiving an education at Norwich Academy from 1837 to 1839. He enlisted in the United States Navy in 1840, as a midshipman. He served on the fleet which supported the landings of Winfield Scott at Vera Cruz in 1847 during the Mexican-American War, and reached the grade of lieutenant by 1855.
At the outbreak of the Civil War, Nelson was sent by President Abraham Lincoln to begin recruiting troops loyal to the Union from Kentucky. Nelson created Camp Dick Robinson in Garrard County, and loyal troops began arriving. For his actions, he was made brigadier general of volunteers on September 16, 1861. Nelson saw his first major combat during the Battle of Shiloh where he lead the 4th Division within the Army of the Ohio under Union General Don Carlos Buell. Nelson enjoyed success when his troops arrived on the field at a critical moment to help repulse a Confederate attack as well as take part in a Union counterattack. After the battle, Nelson continued to lead the 4th Division during Henry W. Halleck’s advance on Corinth along the Chattanooga River. On July 19, 1862, Nelson was promoted to major general, becoming the only naval officer, from either the Union or Confederate navies, to become a full-rank major general of infantry during the Civil War. After he was promoted, Nelson was given command of the Army of Kentucky, which consisted of only two brigades. Nelson led the brigades into Kentucky in order to help stop the Confederate invasion of the state led by Braxton Bragg and Edmund Kirby Smith. On August 30, 1862, Nelson was defeated at the Battle of Richmond, Kentucky, by General Edmund Kirby Smith. The inexperienced Union troops fled from the field of battle, losing a great quantity of equipment, as well as 5,300 men. Nelson was slightly wounded during the battle; however, while recovering he was shot and killed in the Galt House in Louisville, Kentucky. One of Nelson’s fellow officers, Brigadier General Jefferson C. Davis, shot him over an insult.