In the 1977–78 FA Cup, AP Leamington reached the second round; after beating Enderby Town 6–1 in the first round, they were drawn against Southend in the second. The first game at the Windmill Ground was drawn 0–0 and the club lost 4–0 in the replay at Roots Hall. They reached the second round again the next season, eventually losing 1–0 at home to Torquay United. However, a seventh-place finish in the league was enough to secure membership of the new Alliance Premier League, the national non-League division. The club struggled in the new league, finishing in the bottom five in both of their first two seasons, before finishing bottom of the league in 1981–82, which resulted in relegation back to the Southern League Premier Division. Although they won the Premier Division the following season, the club were denied promotion due to the Windmill Ground failing to meet the ground grading criteria.