He was a member of the United States Senate from Maryland, serving from January 1816 until his resignation in December of the same year.

He is best remembered for the phrase, "Millions for defense, but not one cent for tribute"[1] in connection with the XYZ Affair.

At the age of fifteen, Harper joined a volunteer corps of Cavalry and served in the American Revolutionary Army.

He made a surveying tour through Kentucky and Tennessee in 1783, and graduated from the College of New Jersey (now Princeton University) in 1785.

He studied law in Charleston, South Carolina, teaching school at the same time, and was admitted to the bar in 1786.

[7] Through his eldest son Charles, he was a grandfather of Emily Louisa Harper (1835–1908), who married William Clapham Pennington (1829–1913).