Allen was elected to the Northern Ireland Assembly, 1982, representing Londonderry.[2] The following year, he became the Honorary Treasurer of the UUP.[3] In June 1984, Allen was appointed to the prominent role of Chairman of the Devolution Report Committee within the Assembly.
Allen headed the UUP list in Foyle for the Northern Ireland Forum election of 1996, but failed to be elected.[6] At the 1998 Northern Ireland Assembly election, he was again unsuccessful in Foyle, despite coming fourth on first preference votes in the six seat constituency.[7] During this period, he was the chairman of the Foyle Ulster Unionist constituency association.[8]
Allen stood down as UUP treasurer in 2005, citing ill health.[3]