Allen was admitted to the Virginia bar in 1818, and opened his first office at
Campbell Courthouse, Virginia in 1819. He soon moved to
Clarksburg, the seat of
Harrison County where he practiced law in the surrounding counties for seventeen years, after 1830 (until his election as judge) in partnership with
Gideon D. Camden, who also later became a circuit judge. In 1828, voters from the trans-Appalachian counties of Kanawha, Logan, Mason, Cabell, Randolph, Harris, Lewis and Wood elected Allen to represent them in the
Virginia Senate. However, he did not serve a full four year term, because the Virginia Constitutional Convention of 1829-1830 led to a reorganization of districts. His growing district was split, and
William McComas came to represent Kanawha, Mason, Cabell, Logan and Nicholas Counties, and John McWhorter representing Harrison, Lewis and Wood Counties (although both members continued to serve on a part-time basis). In 1832, Allen successfully ran for election to the U.S. Congress, and was a member of the
23rd United States Congress, serving for a year from March 4, 1833 to March 4, 1835. In the 1834 election, voters in Harrison, Lewis and Preston counties elected Allen as their
Commonwealth’s Attorney (state prosecutor). He served for about a year before Virginia's General Assembly elected him as Judge for the seventeenth Circuit in 1836. Allen then moved to
Botetourt County where he held his first court. In December 1840, the Virginia General Assembly elected Judge Allen to the
Supreme Court of Appeals. In 1851 his fellow judges elected him as the court's President during another reorganization. ==Prelude and American Civil War==