William Vincent Allen attended the common schools in Iowa and Upper Iowa University at Fayette, Iowa. He studied law at West Union, Iowa.
Allen practiced in Iowa until 1884, when he moved to Madison, Nebraska. He served as judge of the district court of the ninth judicial district of Nebraska from 1891 to 1893. Allen was the permanent chairman of the Populist State conventions in 1892, 1894 and 1896.
Allen was elected as a Populist to the United States Senate by the Nebraska State Legislature and served from March 4, 1893, to March 4, 1899.
During his term, he served as the chairman of the Committee on Forest Reservations and Game Protection (Fifty-fourth and Fifty-fifth Congresses). Allen championed various bills for public buildings and drought relief along the Missouri River Valley and authored the bill that would establish the 1898 Transport-Mississippi Exposition in Omaha, Nebraska.
Allen was an unsuccessful candidate for reelection in 1899. After that, he was appointed and subsequently elected judge of the district court of the ninth judicial district of Nebraska and served from March 9, 1899 to December 1899, when he resigned to return to the Senate, because he was appointed to fill the vacancy caused by the death of his successor, Monroe L. Hayward. He served from December 13, 1899, to March 28, 1901, when a successor was elected.
He was not a candidate for election to the vacancy. Allen then resumed the practice of law in Madison, where was again elected judge of the district court of the ninth judicial district of Nebraska in 1917 and served until his death.
At various times during his tenure as a United States Senator, Allen served on 10 standing committees and 3 select or special committees. During his first term in the Senate, he chaired the Select Committee on Forest Reservations, and retained his chairmanship when the committee became the standing Committee on Forest Reservations and the Protection of Game during the 54th Congress.
When he returned to the Senate for his second term during the 56th Congress, he was again appointed to this committee, but did not serve as chairman.
On May 9, 1870 William Vincent Allen married Blanche Mott. They had four children.