David Ramsay Clendenin was born in Little Britain, Pennsylvania on June 24, 1830. He moved to Illinois in 1850 and graduated from Knox College (Illinois) in 1854. He was a teacher until the outbreak of the American Civil War when in early 1861 he joined the District of Columbia Militia as a private. On September 18, 1861, he was appointed a captain of the 8th Illinois Cavalry and promoted to major the same day. He was appointed lieutenant colonel of the regiment on December 5, 1862, and brevet colonel on February 20, 1865.
