He was early dedicated to the work of the gospel ministry. In his young manhood he was sent to Ohio, for his education. In 1857, he graduated at Marietta College, and in 1860, at Lane Seminary, Cincinnati. In 1859 he was licensed by Dakota (Indian) Presbytery, and ordained, by the same body, in 1861. The degree of D.D. was conferred upon him by Yankton, (S.D.) college in 1890. He recognized no call to preach the gospel save to the Sioux Indians, and for forty-six years, he has given his whole life zealously to this great work. He has thrown his whole life unreservedly into it. And he has accomplished great things for the Master and the tribe to which he has ministered. In 1860 he established a mission and organized a Presbyterian church of twelve members at Red Wood Agency on the Minnesota. These were both destroyed in the outbreak two years later. He spent the winter of 1862-3, in evangelistic work, among the Sioux, in the prison-camp at Fort Snelling, where 1,500 were gathered under military guard. An intense religious interest sprung up amongst them and continued for months. Young Dr. Williamson so ministered unto them, that the whole camp was reached and roused, and the major part of the adults were led to Christ. Many, including scores of the children of the believers, were baptized. A Presbyterian congregation of more than one hundred communicants was organized. This church was afterwards united with the church of the Prison-pen, at Crow Creek, Nebraska. In 1883, he was appointed superintendent of Presbyterian missions among the Sioux Indians. He has ever abounded in self-sacrificing and successful labors among this tribe. He has organized Nineteen (19) congregations and erected twenty-three (23) church edifices. In twenty-three years he has traveled two hundred thousand miles in the prosecution of these arduous labors. The number of converts cannot be reckoned up. In 1866, he was married to Miss Sarah A. Vannice. To them there have been born four sons and three daughters, who are still living. In 1869 he established the Yankton mission, which has ever since been a great center, moral and spiritual, to a vast region. At the same time he established his home at Greenwood, South Dakota, and from that, as his mission headquarters, he has gone to and from in his great missionary tours throughout the Dakota land. He has, also, abounded in literary labors. For sixteen years he was the chief editor of "Iapi Oayi," an Indian weekly. In 1864, he published "Powa Wow-spi," an Indian Spelling Book, and in 1865, a collection of Dakota Hymns. His greatest literary work, however, was an edition of the "Dakota Dictionary," in 1871, and other later editions.