After progressing through Swansea's Academy, Cornell was included in their senior squad for their 2008 pre-season tour of Spain. He made his professional debut for Swansea on 25 August 2009, in the starting line-up for the League Cup match against Scunthorpe United. Following his release, Cornell joined Oldham Athletic in July 2015 on a one-year contract. In June 2016, he joined Northampton Town on a two-year contract and made his debut on 30 August 2016 against Wycombe Wanderers in the EFL Trophy. In August 2020, Cornell joined Ipswich Town. On 11 June 2021, the club announced that Cornell had left by mutual consent with a year remaining on his contract, and joined Peterborough United, signing a two-year contract. He made his Championship debut in a 3–0 win against Birmingham City. On 27 June 2022, Cornell signed on a two-year deal for Championship team Preston North End, after his contract with Peterborough United expired.