In 1907 he moved to St. Louis, Missouri. In October 1911 he and future Senator Selden P. Spencer founded the law firm Spencer & Donnell. Donnell married Hilda Hays in 1913. They would have two children, Ruth and John Lanier. In 1917 he was president of the Association of Young Republicans of Missouri; in 1918-1920, a member of the executive committee of the Republican State Committee of Missouri; and in 1919, a president of the 28th Ward Republican Club of St. Louis.