He resigned from that position to accept appointment to the administration of President James Monroe, becoming the United States Postmaster General in 1823.

In 1829, President Andrew Jackson appointed McLean as an associate justice of the United States Supreme Court.

It can 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.

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.

In it, a black man named Richard Lunsford, a Kentucky slave, applied for a writ of habeas corpus to obtain freedom from his owner, Thomas D. Carneal.

The most notable portion of this case was McLean's opinion, which highlighted his personal distaste for the institution of slavery: "Were it proper to consider it, the Court, as well as from the principles recognized by our Constitution and Laws, could not hesitate in declaring that SLAVERY [emphasis in original], except for the punishment of crimes, is an infringement upon the sacred rights of man: Rights, which he derives from his Creator, and which are inalienable.

McLean declined both and was instead appointed to the Supreme Court by Jackson on March 6, 1829, to a seat vacated by Robert Trimble.

Though it was "not necessary" to the decision, McLean restated his nationalism by holding that the power to regulate commerce rested exclusively with Congress.

And unless the power be not only paramount, but exclusive, the Constitution must fail to attain one of the principal objects of its formation," McLean wrote.

"[6] All of this he contended while holding the aforementioned view, which demonstrates his tortured reconciliation of two neo-Federalist political biases, which in this case contradict: one favoring an expansive national government, the other condemning slavery.

The most senior member of the Court at the time, McLean began his opinion by weighing in on the debate concerning the nature of the Commerce Clause.

Justice McLean cited Marie Louise v. Marot, an 1835 freedom suit appealed to the Louisiana Supreme Court in which Presiding Judge George Mathews, Jr. stated that "[b]eing free for one moment...it was not in the power of [the plaintiff's] former owner to reduce her again to slavery.

"[9] In opposition to the majority ruling that African-Americans cannot and were not intended to be citizens under the US Constitution, McLean argued that they had already the right to vote in five states.

[10] His strong dissenting views are believed to have forced the hand of Chief Justice Roger Brooke Taney into a harsher and more polarizing opinion than he originally planned.

To the argument that "a colored citizen would not be an agreeable member of society," McLean responded, "This is more a matter of taste than of law."

During the 1820s, McLean was a member of the prestigious Columbian Institute for the Promotion of Arts and Sciences, who counted among their members former presidents Andrew Jackson and John Quincy Adams and many prominent men of the day, including well-known representatives of the military, government service, medical and other professions.