 Towards the end of the 1921 season, Allen was invited to play first-class cricket as an amateur for Middlesex, for whom he qualified by residence. Allen believed that Pelham Warner, who often supported young Middlesex cricketers, encouraged his selection by the club, despite the potential risks to the team's position in the County Championship from playing an inexperienced cricketer. Allen made his first-class debut against Somerset on 21 August 1921 and made one other appearance that season without achieving much in either match.
