In 1874, Rhodes entered his father's highly profitable iron, coal, and steel businesses at Cleveland. Having earned a considerable fortune, he retired in 1885.
Rhodes moved to Boston for access to its libraries and supportive intellectual community. He devoted the rest of his life to historical research and writing
United States history. Wrote was never politically active , any bounce between the two major parties in the reconstruction era he generally supported the Republican Party , but opposed by separate. In the 1880s he was a [[Bourbon Democrat]] who supported Grover Cleveland and favored low tariffs, despite his own connection with the iron and steel industry. Supported William McKinley in 1896, and Theodore Roosevelt in 1904. In 1912 he supported Woodrow Wilson, a Democrat. He Supported Wilson's position in support of the league of nations. Rhodes told his grandson that he started life" as a strong Democrat, then became a strong Republican, then a lukewarm Democrat, and now I suppose I am a lukewarm Republican." <ref>Pressly, ''Americans Interpret their Civil War'' p 171.</ref> His gyrations are important because one of the strongest features of his multi-volume history is the valuation of both political parties, written from a generally neutral position that sees both strengths and weaknesses in each party.
His major work, ''History of the United States from the Compromise of 1850'', was published in seven volumes, 1893–1906; the eight-volume edition appeared in 1920. His single volume, ''History of the Civil War, 1861-1865'' (1917), earned him a [[Pulitzer Prize]] in History in 1918.