Formed in 1905, The Isthmian League soon became the primary amateur league in the South of England.
Starting, as many early leagues did, in a small manner with six clubs, the league had to survive a crisis after only two seasons, when three of the six all left to join the Amateur Football Assocation. Luckily, replacements were found, and a year later, the league expanded to 10 teams.
Since then, the league has usually had many more clubs wanting to join than it has vacancies. For a long period, membership was by invitiation only, many ambitious clubs being unable to convince existing members that they deserved a place at the expense of an existing club. The closed shop situation was not really broken until the 1950s when Barking and Bromley joined. A few new clubs followed, including no less than 4 in 1963, mainly from the Athenian League.
In 1973 the league took a step many had been clamouring for for decades - a second division was formed, again absorbing mainly Athenian League clubs. Since then, further expansion and eventual assimilation into the national pyramid system has seen the structure of the league form into a very strong league of four divisions, the top division being a direct feeder to the national Football Conference.
The amateur status of the league, maintained until the distinction between professional and amateur players was abolished in 1974, meant that the member clubs all had the F.A. Amateur Cup to compete for, and very successful they were too. On no less than 30 occasions did an Isthmian club pick up the Cup, Clapton leading the way with 5, closely followed by Dulwich Hamlet with 4.
Since the re-organisation of non-league football, the clubs of the Isthmian League are split between the F.A. Trophy and the F.A. Vase. Dagenham in 1979-80, followed by Bishop's Stortford the year after were the first Isthmian clubs to take the Trophy at Wembley. Yeading are the only Isthmian club to lift the Vase, while Southall, Oxford City and Berkhamsted Town have all been losing finalists.
Although it's geographic coverage has shifted somewhat due to the re-organisation of the non-league pyramid, the league is currently the highest regional league for a large swathe of the South East of England, covering clubs from Bedford to the South Coast, and from the New Forest eastwards to the North Sea.
Currently the league consists of 3 divisions of up to 22 clubs each, a Premier Division, Divisions One and Two.