Born in Virginia on January 25, 1814, Pierpont was linked with its history for the rest of his life. He grew up in western Virginia, in what is today Marion County, West Virginia, graduated from Allegheny College, and was admitted to the bar in 1841. In 1848 he became the local attorney for the Baltimore and Ohio Railroad.
An active supporter of Lincoln, Pierpont became more involved in politics with the outbreak of the Civil War. When Virginia seceded, he organized a convention of Unionists, which declared that their elected officials had abandoned their posts and elected Pierpont provisional governor of Virginia. A legislature was set up, a new constitution was drafted, and representatives were seated in the Federal Congress. The state adopted the name West Virginia and was admitted into the Union in 1863. When a new governor was elected for West Virginia, Pierpont became governor of the "restored" state of Virginia, those counties occupied by Union troops. The capital, originally in Alexandria, moved in 1865 to Richmond, where Pierpont became governor of the whole state of Virginia.
After he was replaced by a military commander in 1868, Pierpont returned to his law practice in West Virginia. He served one term in the state legislature in 1870 and was collector of Internal Revenue under President Garfield. He died 18 years later in Pittsburgh, Pennsylvania, on March 24, 1899.