A native of Augusta, Georgia, McKinstry moved to Mobile, Alabama, to live with relatives at the age of fourteen. Working odd jobs, McKinstry began reading law while working in the office of John A. Campbell, who would later serve as an Associate Justice of the United States Supreme Court from 1853 - 1861. Soon after, McKinstry was admitted to the bar and began practicing law in Mobile County. Two years later, in 1847, McKinstry was elected colonel of the 48th Alabama Infantry Regiment, 9th Brigade, 4th Division of the State militia. After serving for three years, McKinstry resigned, returned to Mobile, and served as a judge for the city until 1860.
Over the next few years, McKinstry would serve several positions in the military, including serving as colonel of the 32nd Regiment Alabama Infantry during the American Civil War, returning to his law practice in 1864. The following year, in 1865, McKinstry expanded his legal career into politics. He was elected to the Alabama legislature and served as the chairman of the Alabama judiciary committee. He served in this capacity until his election as the third Lieutenant Governor of Alabama in 1872.
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