Though born in New Jersey, John McLean was raised a farmer's son near Lebanon, Ohio. He attended the local school and later was tutored privately. He apprenticed with a Cincinnati lawyer. After his admission to the bar, McLean briefly entered the newspaper publishing business. Thereafter, he was twice elected to the U.S. House of Representatives.
McLean was a loyal Democrat. In return for his support, President James Monroe appointed McLean postermaster general. At the time, this was the largest executive agency. He continued his service under President John Quincy Adams. President Andrew Jackson appointed McLean to the Supreme Court, perhaps hoping to quell his political ambition. But the Court is not a monastery, and McLean repeatedly sought a presidential nod from his Supreme Court perch. For his long years of serve on the Court, McLean's record is extraordinary thin on constitutional issues.