John McKinley, the son of a physician, was born in Virginia and raised in Kentucky. He read law and practiced in Kentucky before moving to Alabama where he was soon drawn to politics rather than law. McKinley was elected to two non-consecutive terms in the U. S. Senate, but he never served the second time. Van Buren chose McKinley to fill one of two newly created seats that enlarged the Court from 7 to 9 members.
McKinley's 15 years on the Court were unproductive. In all, he wrote only twenty opinions, and none touched constitutional matters. He dissented at the same frequency but his was a solo voice, never managing to attract others to his views.