Smithies is a product of the Huddersfield Town academy. Throughout the 09–10 season, he was the only player to play in every league & cup match for Huddersfield. He made 48 appearances and kept 17 clean sheets. He won the club's Young Player of the Year Trophy for his contribution in getting Huddersfield to the finals of the League One playoffs. Smithies joined QPR in 2015 where he played for 3 years before joining Cardiff. In August 2022, he signed for Premier League side Leicester City on a free transfer signing a two-year deal.