West Ham, London born right half Percy Allen served as a Lieutenant in the British Army during the First World War, and played as an amateur for local club West Ham United in wartime league football, featuring in 3 matches during 1917-18 and 1918-19. He began his professional football career with West Ham in the Second Division immediately after the First World War, making his Football League debut against Birmingham in November 1919. He made just 5 appearances during his debut season before breaking into the first eleven on a regular basis in November 1920, and he was an ever present in their 1921-22 campaign as West Ham finished fourth in the division. But he lost his place to Sid Bishop in September 1922 due to the reappearance of a blood ailment which he had suffered from during his time in the Army, and he played in only one early round cup tie as West Ham reached the 1923 FA Cup Final.
Allen played his final competetitive game for The Irons against Barnsley on 17th February 1923, by when he had scored 5 goals in 86 appearances. He was part of the group that travelled to Austria and Hungary in May 1923 but did not feature. He transferred to Lincoln City in late January 1924 and was a virtual ever present during his year and a half at Sincil Bank, missing just one match in their 1924-25 campaign. After 4 goals in 61 appearances he moved to Northampton Town in the 1925 close season and had less than two seasons at The County Ground, scoring 3 times in 45 appearances, joining non league Weymouth in the autumn of 1926. He moved to Southern League club Peterborough & Fletton United in 1927 and finished his career at Stamford Town, who he joined in 1928.