Robert Cooper Grier was born on March 5, 1794 in Cumberland County, Pennsylvania, the eldest of the eleven children of Presbyterian minister Isaac Grier, a member of the Dickinson class of 1788 and his wife Mary Cooper Grier. Schooled by his father, he entered Dickinson at seventeen and finished in one year as a graduate of the class of 1812. Following this, he served briefly as the principal of the Dickinson Grammar School. He then joined his father at his Northumberland Academy, teaching Latin and Greek, and replaced him as headmaster when he died in 1814. He studied the law and was admitted to the Pennsylvania bar in 1817.

He began practice in Bloomsburg and then moved to the county seat at Danville. There he married Isabelle Rose, in 1829, and developed a thriving private practice. Thanks to his staunch Jacksonian views he was named in 1833 as President Judge of the District Court of Allegheny County. He served that bench for thirteen years and developed a deserved reputation as a highly competent judge.

Supreme Court Justice Baldwin had died in 1844, and the two year saga of two presidents (Tyler and Polk), at least three nominees rejected by the Senate, and others (including Buchanan) turning down the nomination, ended when the United States Senate unanimously confirmed Robert Cooper Grier as President Polk's appointment to the Supreme Court on August 5, 1846. He joined another Dickinsonian on the nation's highest court, Chief Justice Roger Brooke Taney. Justice Grier sat during a tumultuous period, rendering opinions in various landmark cases, including the famous Dred Scott Case or Scott v. Sandford in 1857. As the only northern justice to concur with the majority in this case and his adamant support of fugitive slave laws, he was bitterly denounced by abolitionists. He was also accused of engaging in unethical behavior, as a result of intimate correspondence concerning pending legal matters with incoming President James Buchanan, throughout the duration of the Dred Scott case.

Happily for his place in history, his service on the Court over the next decade was unmarked by further stain. On the outbreak of the Civil War, though still a Democrat, he became a staunch supporter of the Union. He cast the deciding vote and delivered the historic opinion on "the Prize Cases" in 1863 which validated the Union's blockade and defined the extent of governmental power in the face of armed rebellion. Serving through the tenure of eight presidents, Grier perhaps stayed too long on the bench. Though he had previously attended every single session during his years as an Associate Justice--he was muscular and over six feet tall--a series of strokes after 1867 reduced his participation to almost nothing and he finally heeded pleas for his retirement on January 31, 1870. He died in Philadelphia on September 25, later that year, aged 76.

His wife Isabelle died in December, 1886. The couple had one daughter, born in 1830.