St Leonards were founded in 1971 as a works team for the local firm STAMCO, with employees initially playing friendlies under the company name. Competitive football soon followed when the club entered the Eastbourne & Hastings Football League for the 1971–72 season.
The club enjoyed a successful rise through local football, reaching the Premier Division by 1976. During this period, they achieved intermediate status and moved into their new ground at Pannel Lane, Pett. In 1982, STAMCO joined the Southern Counties Combination League and won Division Two at the first attempt. After a strong spell in the league, they progressed into the Sussex County League in 1988, finishing as runners-up in Division Three.
The 1992–93 season saw STAMCO finish second in Division Two, earning promotion to Division One. This step up required a ground that met higher grading standards. Club chairman Leon Shepperdson secured a lease on The Firs, formerly the upper pitch of the Pilot Field, and work began to upgrade the facilities in preparation for Division One football.
The club’s success continued as they won the Sussex Senior Cup in 1997 and finished runners-up three times to Peacehaven. Their third runners-up finish secured promotion to the Southern League. However, league rules required the club’s name to include a geographical location, leading to the change to St Leonards Stamcroft—a combination of their principal sponsors, STAMCO and Croft Glass.
St Leonards finished their debut Southern League season as runners-up to Forest Green Rovers, earning promotion to the Premier Division and setting up more fixtures with rivals and neighbours Hastings Town. Unfortunately, the club were relegated after finishing bottom, marking the beginning of a difficult period.
In 1998, the club dropped “Stamcroft” and became St Leonards FC. They struggled in the Southern (later Eastern) Division for five seasons before returning to the Sussex County League for the 2003–04 season. However, mounting tax debts eventually forced the club to fold, withdrawing from the league midway through the campaign.