Allen was born in Ashfield (now
Deerfield, Massachusetts) to Enoch Allen and Mercy Belding Allen. He attended an academy in
Chesterfield, New Hampshire for two years before moving to
Grand Isle, Vermont. He
read law with Elnathan Keyes of Burlington and
Bates Turner of St. Albans. Allen was
admitted to the bar in 1803. He began the
practice of law in Milton, and was the first resident lawyer in Milton. Allen served as a member of the
Vermont House of Representatives from 1810 to 1814, 1816 to 1817, 1822, and 1824 to 1826. He was elected as a
Whig candidate to the
Twenty-fifth Congress, serving from March 4, 1837, until March 3, 1839.{{cite web|url=http://www.govtrack.us/congress/members/heman_allen/400779|title =Rep. Heman "of Milton" Allen While in Congress, Allen served as chairman of the Committee on Expenditures in the Department of the Treasury during the Twenty-third through Twenty-fifth Congresses. He was an unsuccessful candidate for reelection in 1838 to the
Twenty-sixth Congress. After leaving Congress, Allen resumed the practice of law. He was a director of the Lake Champlain Steamboat Company.{{cite web|url= http://beta.worldcat.org/archivegrid/record.php?id=122573701 ==Personal life==