William Mahone (December 1, 1826 – October 8, 1895) was an American civil engineer, railroad executive, and Confederate States Army general. After the Civil War he became a defender of the rights of freedmen and a leader of the Readjuster Party. He represented Virginia in the United States Senate between 1881 and 1887.

As a young man, Mahone was prominent in building Virginia's roads and railroads. As chief engineer of the Norfolk and Petersburg Railroad, he built log-foundations under the routes in the Great Dismal Swamp in southeast tidewater Virginia that are still intact today. According to local tradition, several new railroad towns were named after the novels of Sir Walter Scott, a favorite British/Scottish author of Mahone's wife, Otelia.

In the American Civil War, Mahone was pro-secession and served as a general in the Confederate States Army. He was best known for regaining the initiative at the late war siege of Petersburg, Virginia, while Confederate troops were in shock after a huge mine/load of black powder kegs was exploded beneath them by tunnel-digging former coal miner Union Army troops resulting in the Battle of the Crater in July 1864; his counter-attack turned the engagement into a disastrous Union defeat.

After the war, he returned to railroad building, merging three lines to form the important Atlantic, Mississippi and Ohio Railroad (AM&O), headquartered in Lynchburg. He also led the Readjuster Party, a state political party with a coalition of freemen blacks, Republicans, and populist Democrats. The Virginia General Assembly elected Mahone to the U.S. Senate in 1881.

William Mahone was born at Brown's Ferry near Courtland in Southampton County, Virginia, to Fielding Jordan Mahone and Martha (née Drew) Mahone. Beginning with the immigration of his Mahone ancestors from Ireland, he was the third individual to be called "William Mahone". He did not have a middle name as shown by records including his two Bibles, Virginia Military Institute (VMI) diploma, marriage license, and Confederate Army commissions. Likewise, the General and Otelia's first-born son was christened William Mahone. During similar cultural naming transitions in Virginia, the suffix "Jr." was added to his name later.

The little town of Monroe was on the banks of the Nottoway River about eight miles south of the county seat at Jerusalem, a town which was renamed Courtland in 1888. The river was a vital transportation artery in the years before railroads, and later highways served the area. Fielding Mahone ran a store at Monroe and owned considerable farmland. He also enslaved several people for their forced labor. The family narrowly escaped the killings of local whites during Nat Turner's slave rebellion in 1831.

The local transportation shift in the area was from the river to the new technology emerging with railroads in the 1830s. In 1840, when William was 14 years old, the family moved to Jerusalem, where Fielding Mahone purchased and operated a tavern known as Mahone's Tavern. As recounted by his biographer, Nelson Blake, the freckled-faced youth of Irish-American heritage gained a reputation in the small town for both "gambling and a prolific use of tobacco and profanity".

Young Billy Mahone gained his primary education from a country schoolmaster but with special instruction in mathematics from his father. As a teenager, for a short time, he transported the U. S. Mail by horseback from his hometown to Hicksford, a small town on the south bank of the Meherrin River in Greensville County, which later combined with the town of Belfield on the north bank to form the current independent city of Emporia. He was awarded a spot as a state cadet at the recently opened Virginia Military Institute (VMI) in Lexington, Virginia. Studying under VMI Commandant William Gilham, he graduated with a degree as a civil engineer in the Class of 1847.