associate justice of the United States Supreme Court (1846–70).

Educated at home, Grier took over his father's educational academy in Northumberland, Pa., at the age of 21 and taught Latin, Greek, mathematics, astronomy, and chemistry at the same time that he was studying law. After being admitted to the bar in 1817 he practiced successfully in the towns of Bloomsburg and Danville and was appointed president of the District Court of Allegheny County in 1833. In 1846 he was appointed to the U.S. Supreme Court by Pres. James K. Polk.

Despite his early associations with the Democrats, Grier was a staunch Unionist during the Civil War. Although he concurred in the proslavery Dred Scott decision (1857), he spoke for the court in the 1863 prize cases, which upheld the power of the president to proclaim a blockade of Confederate ports and to seize neutral shipping.