Mills made his Forest Green Rovers debut, starting the whole game, and helped the club kept a clean sheet, in a 1–0 win against Oldham Athletic in the opening game of the season. He then started in the next five matches for Forest Green Rovers before being sent–off for a second bookable offence against Bradford City on 24 August 2019 and then suffered a hamstring injury during a 2–0 loss against Newport County on 31 August 2019. After being sidelined for weeks, it wasn't until on 28 September 2019 when Mills returned to the starting line–up against Salford City and kept a clean sheet, as the club won 4–0. However, his return was short–lived when he suffered a hamstring injury in the 10th minute and was substituted, as Forest Green Rovers won 3–1 against Crawley Town on 5 October 2019. It wasn't until on 12 November 2019 when Mills returned to the starting line–up against Walsall, captaining the club, as they lost 6–0. Following this, he regained his first team place, playing in the centre–back position. However, Mills soon lost his first team place and was demoted to the substitute bench. This last until the season came to a premature end due to the global pandemic COVID-19. At the end of the 2019–20 season, he went on to make twenty–one appearances in all competitions.