He graduated MB and ChB in 1914 and joined the Royal Army Medical Corps a few days after the United Kingdom declared war on Germany.

[1] In September 1916 Allen, by then promoted to captain, was awarded the Military Cross for actions on unspecified dates: For conspicuous gallantry and devotion to duty.

[2]On 3 September 1916 Allen was attached to the 246th (West Riding) Brigade, Royal Field Artillery, near Mesnil, France, when the following event took place for which he was awarded the Victoria Cross "for most conspicuous bravery and devotion to duty": When gun detachments were unloading H.E.

Captain Allen saw the occurrence and at once, with utter disregard of danger, ran straight across the open, under heavy shell fire, commenced dressing the wounded, and undoubtedly by his promptness saved many of them from bleeding to death.

During an intense bombardment of a town with high explosive and gas shells, he left the Advance Dressing Station to search for wounded men.