 Following AFC Wimbledon's promotion to the Football league Kiernan made his football league debut on 24 September 2011 when he came on as a substitute for Luke Moore in a 2–1 win over Bradford City in League Two.On 27 January 2012, Kiernan signed for Braintree Town on a one-month loan deal, making four appearances in total.Kiernan scored his first goal in professional football in the First Round of the League Cup against Stevenage on 14 August 2012.