John Thomson Mason Jr. (May 9, 1815 – March 28, 1873) was a U.S. Congressman from Maryland, representing the sixth district from 1841 to 1843.

Born at the Montpelier estate near Hagerstown, Maryland, Mason was educated by a private tutor and graduated from Princeton College in 1836. He studied law, was admitted to the bar, and commenced practice in Hagerstown in 1838.

Mason later served as a member of the Maryland House of Delegates in 1838 and 1839, and was elected as a Democrat to the Twenty-seventh Congress, serving one term from March 4, 1841, to March 3, 1843. He was a judge of the Maryland Court of Appeals from 1851 to 1857, and afterwards a collector of customs at Baltimore, Maryland, from 1857 to 1861. He moved to Annapolis, Maryland, and served as Secretary of State of Maryland from 1872 until his death in 1873.

Mason died on March 28, 1873, in Elkton, Maryland at age 57. He is interred in Rose Hill Cemetery (Maryland) in Hagerstown, Maryland.

Mason married Margaret Augusta Cowan in Alleghany City, Pennsylvania on December 14, 1842. The couple had four children:

John Thomson Mason Jr. was a grandnephew of George Mason (1725–1792); grandson of Thomson Mason (1733–1785); nephew of Stevens Thomson Mason (1760–1803); son of John Thomson Mason (1765–1824) and Elizabeth Beltzhoover Mason (1781–1836); second cousin of Thomson Francis Mason (1785–1838); first cousin of Armistead Thomson Mason (1787–1819), John Thomson Mason (1787–1850), and James Murray Mason (1798–1871); and first cousin once removed of Stevens Thomson Mason (1811–1843).