William Hemsley Emory was born on September 7, 1811 in Queen Anne's County, Maryland, the son of Thomas and Anna Maria (Hemsley) Emory. Emory was graduated from the United States Military Academy at West Point in 1831, where he was know as "Bold Emory," and promoted to brevet second lieutenant, 4th Artillery. He resigned from the service in 1836. Two years later, upon the reorganization of the army, he re-entered the service and was commissioned first lieutenant in the Topographical Engineers. He also married Matilda Wilkins Bache, a great-granddaughter of Benjamin Franklin in May of that year. Emory served as principal assistant on the Northeastern boundary survey between the United States and Canada from 1844 to 1846. At the outbreak of the Mexican-American War in 1846, he became the chief engineer officer and acting assistant adjutant-general of the Army of the West and subsequently as a lieutenant-colonel of volunteers in Mexico.
While with the Army of the West, he won two brevets for distinguished service at the battles of San Pasquale, San Gabriel, and the Plains of Mesa. After the war, he became chief astronomer for the team appointed to determine the boundary line between California and Mexico (1848-1853). During the course of these duties, in 1851, he was promoted to captain. In 1854, he was appointed commissioner and astronomer with full powers, under the Gadsen Treaty. When the army was again reorganized in 1855, he was promoted to major in the 2nd Cavalry, a newly-formed regiment. After completing the work connected with the Gadsen boundary surveys in 1857, he was brevetted lieutenant-colonel in recognition for his service. Moved by familial concerns at the outbreak of the Civil War, Emory resigned his command on May 9, 1861, an action he immediately regretted. He tried to intercept the written dispatch of his resignation before it could be delivered, but failed in his efforts. Although within a week after this action, he was commissioned major general of the newly organized 3rd, later 6th Cavalry, it took several months for his reinstatement to be formally recognized. Not until General Winfield Scott, Lieutenant A. V. Colburn, and Emory himself testified to the Secretary of War and the Senate regarding his character, military performances, and loyalty to the Union, was his resignation formally rescinded and his reinstatement and current rank confirmed. Emory was appointed brigadier-general of volunteers in 1862 and served with distinction as brigade, division, and corps commanders. He received four brevets for separating the wings of the Confederate Army at Hanover Court House, destroying the railroad bridges between Hanover Junction and Chickahominy River, and driving the enemy out of Ashland Virginia. In 1865, he was commissioned major-general of volunteers and commanded the Department of West Virginia until mustered out of the volunteer service in January 1866. He commanded successively the Department of Washington, District of the Republican (1869-1871), and the Department of the Gulf (1871-1875) and retired with the rank of brigadier general on July 1, 1876 after forty-five years of service. He died December 1, 1887 in Washington, D. C., and was buried in Congressional Cemetery.