McLean was born in Morris County, New Jersey, the son of Fergus McLean and Sophia Blackford. After living in a succession of frontier towns, Morgantown, Virginia; Nicholasville, Kentucky; and Maysville, Kentucky; in 1797 his family settled in Ridgeville, Warren County, Ohio. It was here that McLean received his formal education in the classical style and that his interest in law began to develop. Going on to graduating from Harvard in 1806. It can also be argued that his anti-slavery views also began to form at this time, given his upbringing as an evangelical Methodist with a focus on egalitarianism. His brother William was also a successful Ohio politician. His brother Finis McLean was a United States Representative from Kentucky.

He read law and was admitted to the bar in 1807. That same year he founded The Western Star, a weekly newspaper at Lebanon, the Warren County seat. In 1810 Mclean transferred ownership of the Star to his brother Nathaniel and hung up his shingle, beginning to practice law as an individual lawyer for the first time. He was elected to the U.S. House for the Thirteenth and Fourteenth Congresses, serving from March 4, 1813, until he resigned in 1816 to take a seat on the Ohio Supreme Court, to which he had been elected on February 17, 1816, replacing William W. Irvin.

He resigned his judgeship in 1822 to take President James Monroe’s appointment to be Commissioner of the General Land Office, serving until 1823, when Monroe appointed him United States Postmaster General. McLean served in that post from December 9, 1823, to March 7, 1829, under Monroe and John Quincy Adams, presiding over a massive expansion of the Post Office into the new western states and territories and the elevation of the Postmaster Generalship to a cabinet office.

While Postmaster General, McLean supported Andrew Jackson, who offered him the posts of Secretary of War and Secretary of the Navy. McLean declined both and was instead appointed to the Supreme Court by Jackson on March 6, 1829, to a seat vacated by Robert Trimble. McLean was confirmed by the United States Senate on March 7, 1829, receiving his commission the same day.

Known as “The Politician on the Supreme Court,” he associated himself with every party on the political spectrum, moving from a Jackson Democrat, to the Anti-Jackson Democrats, the Anti-Masonic Party, the Whigs, the Free Soilers, and finally the Republicans. McLean was touted as a potential Whig presidential candidate throughout the 1830s-40s. President John Tyler offered him the post of Secretary of War, but he declined. Because of his anti-slavery-extension positions, he was considered by the new Republican party as a presidential candidate in 1856, but the nomination went to John C. Frémont. McLean sought the presidency again in 1860 despite turning 75 that March. He won twelve votes on the first ballot at the Republican convention; Abraham Lincoln ultimately was nominated.