Sadler made his debut on 14 August, starting in a League Cup match as a left back against Championship side Millwall which ended 2–2, with Crawley winning 4–1 on penalties. Four days later, on 18 August 2012, Sadler made his league debut for Crawley Town, in the opening game of the season, in a 3–0 win over Scunthorpe United. Sadler then scored his first goal for the club on 5 March 2013, in a 1–1 draw against Carlisle United. In his first season at Crawley Town, Sadler went on to be an ever-present player in the League One season to play every minute of every game for his club, as he helped the club finished tenth place.