Cushman Kellogg Davis relocated with his family from New York state to Waukesha, Wisc., as a young boy. After graduation from the University of Michigan in 1857 he studied law and opened a practice in his home town. Commissioned first lieutenant in the 28th Wisconsin Infantry in 1862, Davis's Civil War career included service as judge-advocate and adjutant-general on the staff of Gen. Willis Gorman. In 1864 he returned to Wisconsin, married, and relocated to St. Paul, Minn., to join his former commanding officer in law practice.
Davis's political career began with 1867 election to the state legislature, and he subsequently served as United States district attorney from 1868 to 1873. Though a Republican, Davis supported the Granger movement, speaking out against business abuses by the railroads, and gaining a reputation that won him the governor's seat in 1873 due to his influence over the Granger vote. Losing a bid for U.S. Senator in 1875, he settled back into legal work until 1887, when a successful Senate race resurrected his political career. Davis served in the Senate, keeping a law practice on the side, for the rest of his life.
As a Senator he concentrated on committee work and focused primarily on pensions and foreign relations. He went to Paris as a peace commissioner following the Spanish American War, and used the position to advance American expansionism. Although his early political career had been characterized by anti-monopoly and pro-agriculture sentiments, Davis came to espouse the basic ideals of the Republican party, though moderated by a sympathy for farmers in his agricultural state. He was an ardent imperialist, an advocate of strict law and order in the face of labor unrest, and a tariff advocate. An early marriage, to Laura Bowman of Waukesha, ended in divorce, and in 1880 Davis married Anna Malcolm Agnew of St. Paul. They had no children.