Allen began his career at Brentford joining the club aged 14, whilst his father was first-team manager of the club. Allen progressed into the youth team team but was not offered a professional contract at the end of his 2 year scholarship. During the summer he spent sometime at a Swedish division 4 club Ånge IF. After that Allen joined Dagenham & Redbridge youth academy, playing for Billericay Town on dual-registration, before signing at Notts County for the start of the 2011–12 season. He made his debut in a 2–0 defeat to Preston North End at Deepdale on 27 August 2011, before he was replaced by Alan Judge after 67 minutes.
In May 2012 he was released by the club, along with 12 other players. Two months later he was signed by Football League Two side Gillingham on an initial one-year deal, with the option of extending for a further season. He made his debut for Gillingham on 18 August in 3–1 win against Bradford City