Having begun studying medicine with the mining company's resident physician, in 1842 he accepted $200 from his father, intending to sail to the United States and pursue a career in the medical field.

McLean secured a position as a clerk in a drug store, and continued to learn the medical profession, including courses at the University of Pennsylvania.

[1] In 1850, he moved to New Orleans, Louisiana, where his success at turning a profit by purchasing and then re-selling the only supply of turpentine then available in the city led to his taking charge of finances for the Narciso López expedition that attempted to liberate Cuba from control of Spain.

Mclean was elected as a Republican to the Forty-seventh Congress to fill the vacancy caused by the death of Thomas Allen and served from December 15, 1882, to March 3, 1883.

As a devout Methodist, he opposed violence, and hoped that weapons capable of mass killing would prove to be so terrible that nations would be discouraged from going to war.