William Pinkney was born in Annapolis, Maryland, on March 17, 1764. He studied law in the office of Judge Samuel Chase in Baltimore and was admitted to the bar in 1786. He was a member of the Constitutional Convention in 1788 and voted against ratification. He was a delegate to the Maryland House of Representatives from 1788 to 1792 and served as a member of the State executive council from 1792 to 1795.
In 1796, Pinkney served as a commissioner under Jay's Treaty to settle U.S. claims against Great Britain. He was sent to England in 1806 with James Monroe on a similar mission. He returned to the United States, settled in Baltimore, and in 1811, was elected to the State senate. On December 11, 1811, Madison appointed Pinkney Attorney General of the United States. He served until February 10, 1814. He commanded a battalion of riflemen in the War of 1812, and was wounded at Bladensburg, Maryland. He served in Congress in 1815 and 1816. From 1816 until 1818, he was Minister to Russia and Envoy to Naples. On his return in 1819, he was elected to the United States Senate. He died in office on February 25, 1822, in Washington, D.C.