John Thomson Mason (15 March 1765 – 10 December 1824) was an American lawyer and Attorney General of Maryland in 1806.
Mason was born on 15 March 1765 at Chopawamsic in Stafford County, Virginia. He was the third child and youngest son of Thomson Mason and his wife Mary King Barnes.
Mason operated a plantation in what was then Washington County, Maryland near Elizabethtown (now Hagerstown using enslaved labor.
Admitted to the Maryland bar, he attained high rank, but twice declined the office of United States Attorney General when it was offered by Presidents Thomas Jefferson and James Madison. He then served as Attorney General of Maryland in 1806. He was also one of six judges appointed to a newly restructured court of appeals by Governor Robert Bowie on 19 January 1806, but declined the appointment. Mason ran for one of Maryland's seats in the United States Senate in 1816, but lost.
Mason married Elizabeth Beltzhoover in 1797. He and Elizabeth had seven children:
Mason died on 10 December 1824 at the age of 59. Mason was interred at his Montpelier estate in Clear Spring, Maryland.
John Thomson Mason was a nephew of George Mason (1725–1792); son of Thomson Mason (1733–1785); brother of Stevens Thomson Mason (1760–1803); half-brother of William Temple Thomson Mason (1782–1862); first cousin of George Mason V (1753–1796); first cousin once removed of Thomson Francis Mason (1785–1838), George Mason VI (1786–1834), Richard Barnes Mason (1797–1850), and James Murray Mason (1798–1871); uncle of Armistead Thomson Mason (1787–1819) and John Thomson Mason (1787–1850); father of John Thomson Mason Jr. (1815–1873); and great uncle of Stevens Thomson Mason (1811–1843).