In 1911 Forest Green merged with Nailsworth to form Nailsworth & Forest Green United, continuing to play at the Lawn Ground and in the Stroud & District League; the new club won the league, losing only one match all season. They entered a team into the Dursley & District League in 1912–13, but withdrew from the Stroud & District League after only four matches. After World War I the club rejoined the league and the 1919–20 season saw them finish joint top of the table with Chalford and Stonehouse. The league subsequently held a draw to decide the championship, with Forest Green drawn against Stonehouse in a semi-final, with the winner to play Chalford for the title. However, Stonehouse beat Forest Green 3–2 in the semi-final match. In 1920 the club also entered a team into the North Gloucestershire League and went on to win both leagues, as well as the Northern Junior Cup. They repeated the double league championship the following season. They were founder members of the Gloucestershire Northern Senior League in 1922, but left the league at the end of the 1922–23 season to return to the Stroud & District League.