Darnall was born on his father's farm in the Cottage Hill community near McKinney, in Collin County, Texas. He was the eldest of the seven children of Reverend Joseph Rogers Darnall, minister of the Christian Church, and Mary Ellen (Thomas) Darnall.
Darnall lived in Milford, New Jersey from 1889 to 1896 and established his first practice there. His later assignments were to stations in Texas – Fort Clark at Brackettville, and Fort McIntosh at Laredo. During the Spanish–American War (1898), Darnall served in Cuba. Later, he served as an operating surgeon and pathologist aboard the hospital ship USS Relief in the Philippines and commanded the hospital at Iloilo. He was one of the few medical officers that accompanied the Allied Forces during the Boxer Rebellion in China.[2]
In 1902, Darnall returned to Washington, D.C., and served as secretary of the faculty and instructor for sanitary chemistry and operative surgery at the Army Medical School. It was while a major and a professor of chemistry there that he demonstrated the value of anhydrous, liquefied chlorine in purifying water. His 1910 invention, the mechanical liquid chlorine purifier (known as a "chlorinator"), was the prototype of the technology that is now applied to municipal water supplies throughout the world. Darnall also devised and patented a water filter that was used by the Army for many years. Major William Lyster further adapted the process of water chlorination to field use by inventing a method to apply sodium hypochlorite in a cloth bag, known as a Lyster bag.
Later career
Darnall was promoted to colonel in 1917. During World War I, Darnall's talents for business and organization were recognized and he was assigned to the Finance and Supply Division in the Office of The Surgeon General (OTSG). After the war, he served as department surgeon in Hawaii. In 1925, he returned to the OTSG as executive officer. In November (or December 5?), 1929, he was promoted to brigadier general and became the commanding general of the Army Medical Center, a post he held until he retired on December 31, 1931.
Darnall died on 18 January 1941 at Walter Reed General Hospital, Washington, D.C. Six days earlier, his wife had died at the family home in Washington. They left three sons, Joseph Rogers (1894–1976), William Major (b. 1895), and Carl Robert (b. 1904), all of whom served in some capacity in the Army. He is buried in the Arlington National Cemetery in Arlington, Virginia.
