William Allen, of Quaker ancestry, was the son of a Revolutionary War officer. He was born in Edenton, North Carolina, on December 27, 1803, and when he was 16 journeyed alone to Ohio to be with his sister. He hoped to make his future in the West and began by studying at the Chillicothe Academy for two years, supplementing this education with reading. He then studied law with Colonel Edward King and at age 21 was admitted to the bar. Allen rode the circuit and soon became well known.

Allen's skill in debate and his overall demeanor inspired the Jackson Democrats of his district to nominate him as their congressional candidate; he won, even though the district was Republican. He served only one term in the House of Representatives. Although not reelected he was subsequently appointed by the governor to complete Thomas Ewing's term as a United States Senator. He was reelected to a second term. During this second term Allen coined the political slogan "Fifty-Four/Forty or Fight!," which indicated his strong support of United States territorial rights in Oregon. Allen also favored the annexation of Texas. Salmon P. Chase defeated Allen in his reelection bid, but this did not silence "Earthquake Allen."

He became an outspoken critic of Lincoln and was an anti-war Democrat. Allen made a political comeback in 1873 when he was elected governor of Ohio. He served one term and died at his estate, "Fruit Hill," on July 11, 1879.