Born probably on his father’s estate near Leonardtown, St. Mary's County, Maryland, he pursued an academic course in England. Key eventually returned to Maryland and engaged in farming, he then began to study law and was later admitted to the bar and practiced.
Key served in the Maryland House of Delegates in 1773, he then became member of the committee of correspondence for St. Mary's County, in 1774. He again served as a member of the House of Delegates during 1779 - 1790. Key was then elected to the Second Congress, and represented the 1st Congressional district of Maryland from March 4, 1791 – March 3, 1793. Key then returned to the House of Delegates in 1795 and 1796 where he served as speaker.
Key died in Chaptico, Maryland in St. Mary's County in 1820 and is probably interred in the churchyard at Chaptico.