John Oatman Dewees, cattleman, son of Thomas and America (Oatman) Dewees, was born in Putnam County, Illinois, on December 30, 1828. In 1849 he moved to Hallettsville, Texas, with his family and, in partnership with his father and brother Thomas, operated a stock farm near Bastrop. In 1854 he moved to Seguin and in 1857 to Live Oak County; he raised livestock in both places on free range. By the time of the Civil War he owned 1,600 cattle. In 1862 he joined Company B of Col. Peter C. Woods's Thirty-second Texas Cavalry, with which he served throughout the conflict; he reportedly participated in more than thirty skirmishes, including Blair's Landing and the battle of Yellow Bayou.
After he was paroled in 1865 Dewees returned to Texas and with borrowed money bought pastureland in Wilson County, on which he raised cattle. In 1871, in association with James F. Ellison, he drove 2,000 cattle to Kansas and sold them profitably. The two men soon thereafter formed a partnership: Dewees bought Texas cattle, and Ellison oversaw their delivery and marketing at northern railheads, ranges, and Indian reservations. By 1882, when the partnership was dissolved because of Ellison's financial reverses, the two had delivered more than 400,000 cattle to the northern market and ranked among the state's leading drovers. Afterward Dewees ranched on 60,000 acres that he partly owned and partly leased in Wilson, Karnes, and Atascosa counties. From 1876 to 1899 he lived in San Antonio and traded livestock there. Dewees was a Mason. He married Anna Irvin of Guadalupe County in 1873, and they had one daughter. When Dewees died in San Antonio on June 10, 1899, his estate was valued in excess of $300,000.