Born: 1819-03-12 Canajoharie, New York
Diefendorf moved to Springfield, Illinois, in November 1840. In January 1842, he married Martha Ann Cutter, who died six weeks after their nuptials. Around this time, Diefendorf became deputy clerk of the Sangamon County Circuit Court. In this capacity, he became acquainted with Abraham Lincoln and his partners. Diefendorf served as deputy clerk until June 1846, when, at the commencement of the Mexican War, he enlisted in Company D of the 4th Regiment of Illinois Volunteers. He received a commission as first lieutenant and travelled with the 4th Regiment to Mexico. In March 1847, he received a commission as second lieutenant in the 16th U.S. Infantry and completed the war under that rank. In October 1848, Diefendorf entered into partnership with Samuel V. Niles, another officer from the 16th U.S. Infantry, and established an office in Washington, DC, to settle claims for bounty lands, extra pay, and pensions due soldiers who participated in the war. Diefendorf moved to Cincinnati, Ohio, where in October 1848, he married Caroline Cutter, with whom he had three children, all who died in infancy. In August 1850, Diefendorf moved to Weston, Platte County, Missouri. A year later, he won election as clerk of the Court of Common Pleas for Platte County. Diefendorf next served two years as a clerk in the office of the surveyor-general of the Kansas Territory. In 1854, he joined other Missourians in creating the Leavenworth Town Company, and was one of the original proprietors of Leavenworth, which the company laid out in 1854. In 1856, he moved to the Kansas Territory, and in 1857, he became a resident of Leavenworth. He worked as a war claims agent through the Civil War.
Illinois Statewide Marriage Index, Sangamon County, 1 January 1842, Illinois State Archives, Springfield, IL; For Lincoln's cases in which Diefendorf was a participant, search "Diefendorf, Oliver," Martha L. Benner and Cullom Davis et al., eds., The Law Practice of Abraham Lincoln: Complete Documentary Edition, 2d edition (Springfield: Illinois Historic Preservation Agency, 2009), http://www.lawpracticeofabrahamlincoln.org; Sangamo Journal (Springfield, IL), 30 July 1846, 2:6; 18 March 1847, 3:1; Illinois Journal (Weekly) (Springfield), 3 October 1848, 3:5; Jesse A. Hall and Leroy T. Hand, History of Leavenworth County Kansas (Topeka, KS: Historical Publishing, 1921), 118; Leavenworth City Directory, and Business Mirror, for 1863-1864 (Leavenworth, KS: Hume & Prescott, 1863), 54; Leavenworth City Directory and Business Mirror for 1865-1866 (Leavenworth, KS: Braunhold & Crowell, 1865), 63; John Carroll Power and S. A. Power, History of the Early Settlers of Sangamon County, Illinois (Springfield, IL: Edwin A. Wilson, 1876), 250.