Gilmore was born in Bedford County, Virginia, son of Dr. Eli Gilmore and Clarissa Mosby Clayton, sister of a prominent Mississippi judge, later a member of the Confederate Congress, Alexander Mosby Clayton.

The family moved to Israel Township, Preble County, Ohio in 1825, where the father became a prominent physician.

[1] He began study of law in 1844 in the office of Thomas Millikin in Hamilton, Ohio, and continued with J.S.

[1] Gilmore was Prosecuting Attorney of Preble County for two terms, and was elected to the Common Pleas bench in 1857.

[3] In 1874, Gilmore was selected at the state Democratic Party convention to run for a seat on the Ohio Supreme Court, and later that year defeated Republican incumbent Luther Day for a five-year term.