The 2011–12 season was the 97th season of the Isthmian League, which is an English football competition featuring semi-professional and amateur clubs from London, East and South East England.
[1] The Premier Division consisted of 22 clubs, including 17 clubs from the previous season, and five new clubs: Billericay Town won the division and were promoted to the Conference South along with play-off winners AFC Hornchurch, while Lowestoft Town lost their second consecutive play-off final.
Great Wakering Rovers finished bottom of the table and were the only relegated club, while Ware were reprieved due to clubs folding higher up in the pyramid.
After defeating Godalming Town in the semi-final, Bognor Regis Town gained revenge on their semifinal rivals from the previous year, Dulwich Hamlet, 1–0 and joined Whitehawk in the Premier Division.
Croydon Athletic resigned from the league in the middle of the season, so there was only one relegation place, taken by Whyteleafe.