Osborn lived in Sault Ste. Marie, Michigan for many years where he ran a newspaper, The Sault News, and also began his involvement in politics. In 1889, he was appointed Postmaster of Sault Ste. Marie and, in 1895, state Fish and Game Warden. In 1898, Governor Hazen S. Pingree appointed Osborn Commissioner of Railroads, a position in which he served from 1899 to 1903. After selling out his newspaper, he and Walter J. Hunsaker bought The Saginaw
In 1910, Osborn was elected the 27th Governor of Michigan and served from 1911 to 1913. His tenure as governor was focused on reforms as the state deficit was eliminated; a workman's compensation bill was sanctioned
After his term as governor, Osborn traveled the world and came back for another attempt to become governor again and unseat his successor, Democrat Woodbridge Nathan Ferris, but was unsuccessful. In 1918 he was a candidate in the primary for United States Senator from Michigan, being defeated by fellow Republican Truman H. Newberry. He supported Woodrow Wilson's League of Nations, and urged participation in world affairs during the 1920s and 1930s despite the consensus of isolationism during those years. Osborn met Stellanova Brunt in 1924, and she took a job as his researcher and secretary. In 1928, he was a candidate for the Republican nomination for U.S. Vice President, yet was defeated by Charles Curtis, who would win with Herbert Hoover as president. In 1930, he was again a candidate in the primary for the U.S. Senate, being defeated by fellow Republican James Couzens. In 1931, Chase and Lillian Osborn legally adopted 37-year-old Stellanova, and she changed her last name to Osborn. In 1934, he was elected chairman of the Michigan Republican Party. In 1936, he was a candidate for Presidential Elector to elect Alfred Landon, who would lose to Franklin Roosevelt. In 1939, he met with Roosevelt to lobby for construction of the Mackinac Bridge, a project that would be completed in 1957 after the deaths of both Osborn and Roosevelt.
During his two-year term as governor he supported or passed legislation on the following issues:
* Woman's suffrage (not incorporated as law in Michigan until 1918)
* Prohibiting wholesale distributors or producers of liquor from owning saloons
* Teaching of agriculture in public schools and state aid for country agricultural schools
* An unsuccessful attempt to create a State Department of Agriculture
* Controls on banks
* Legal use of convict labor to build roads and a general road improvement program
* Child and woman labor laws
* Workman's compensation
* Transformed Michigan's half-million dollar debt into a surplus of a half-million in two years by abolishing unnecessary offices and employees and by carefully managing state business.