Cox was born in Montreal, Canada, to American parents, Jacob Dolson Cox and Thedia Redelia Kenyon Cox. Cox married Helen Clarissa Finney, whom he met at Oberlin College in Ohio.He became superintendent of the Warren, Ohio, school system as he studied law and was admitted to the bar in 1853. As a strong abolitionist, in 1855 he helped to organize the Republican Party in Ohio and stumped for its candidates in counties surrounding Warren. He entered the Ohio State Senate in 1860 and formed a political alliance with Senator and future President James A. Garfield, and with Governor Salmon P. Chase. While in the legislature, he accepted a commission with the Ohio Militia as a brigadier general and spent much of the winter of 1860–61 studying military science.After Cox fought in the Civil War he became the Governor of Ohio from 1866-1867. He was appointed Secretary of the Interior by Ulysses S. Grant upon his inauguration in March 1869, serving until November 1870.During his later years, Cox was a prolific author. His works include Atlanta (published in 1882); The March to the Sea: Franklin and Nashville (1882); The Second Battle of Bull Run (1882); The Battle of Franklin, Tennessee (1897); and Military Reminiscences of the Civil War (1900).Cox died on summer vacation at Gloucester, Massachusetts. He is buried in Spring Grove Cemetery, Cincinnati.