Oglesby, the son of Jacob and Isabella Watson Oglesby, was born into a farming family, and his father was a member of the Kentucky legislature. Orphaned in 1833 following the deaths of his parents (as well as three siblings) from cholera, he was taken to Decatur, Illinois, by an uncle. Admitted to the bar in 1845, Oglesby practiced law in Sullivan, Illinois, until 1846, when the Mexican War (Mexican-American War) broke out. During the war he served with the 4th Illinois Volunteers, fighting in the Battle of Cerro Gordo (Cerro Gordo, Battle of). In 1849 he was drawn to California by the gold rush; he returned to law practice in Decatur in 1851. An abolitionist, he joined the Republican Party and ran unsuccessfully for the U.S. House of Representatives in 1858. Elected to the Illinois state Senate in 1860, Oglesby served only one term before resigning to serve in the American Civil War as colonel of the 8th Illinois Volunteers. He was promoted to major general in 1862 and was elected governor of Illinois in 1864. He returned to law practice in 1869 but was once again elected governor in 1872. Shortly after taking office in 1873, he was elected to the U.S. Senate, in which he served until 1879. He served an unprecedented third term as governor of Illinois from 1885 to 1889. The city of Oglesby, Illinois, was named for him.