England Isthmian Premier Division

Before the Isthmian League was formed, there were no leagues in which amateur football clubs could compete, only knock-out cup competitions. Therefore, a meeting took place between representatives of Casuals, Civil Service, Clapton, Ealing Association, Ilford and London Caledonians to discuss the creation of a strong amateur league. All the clubs supported the idea and the Isthmian League was born on 7 March 1905. Membership of the league was through invitation only. The league was very strongly dedicated to the cause of amateurism in sport; consequently, the champions of the league did not even receive a trophy or medals; the league's Latin motto was honor sufficit ('honour is sufficient').

Thus, those clubs less able to compete financially gravitated to it rather than to the Southern League, which attracted clubs with ambition and money. Although the Isthmian League established itself as one of the strongest amateur leagues in the country, routinely providing the winners of the FA Amateur Cup, it was still regarded as being at a lower level than the Southern League, which had developed into the top regional semi-professional league. By 1922 the Isthmian League had fourteen clubs and over the next five decades, only a few new members were admitted, mainly to fill vacancies left by clubs leaving the league. Most new Isthmian League members joined from the Athenian League, which was similarly dedicated to amateurism.

The Isthmian League was most likely named after the ancient Isthmian Games, with the later Athenian League, Corinthian League, Delphian League, Spartan League, Parthenon League and Aetolian League all adding a Classical Greek flavour to amateur football competition. In 1962 an 'all-star' team from the Isthmian League entered the 1962 Ugandan Independence Tournament, drawing both their games versus Kenya and Ghana.

The league finally began to permit professionalism in the mid-1970s when the Football Association abolished the long-standing distinction between amateur and professional status with effect from the 1974–75 season. A second division of sixteen clubs was formed in 1973 and a third division followed in 1977. However, the league still remained officious and refused to participate in the formation of the Alliance Premier League in 1979 and whilst two Isthmian clubs, Enfield and Dagenham, defected to the APL in 1981, it was not until 1985 that the Isthmian League champions were given a promotion place to the newly renamed Football Conference. The reward of promotion into the Conference means that, since 1985, no team has won the league champions title two seasons in succession (as had happened on 22 occasions previously). The Athenian League disbanded in 1984 when the Isthmian League Second Division split into North and South Divisions. These were restructured yet again to Second and Third Divisions in 1991.

In 2002, the league was restructured once more, with the First and Second Divisions merging to become Division One North and Division One South (later renamed simply the North and South divisions), and the Third Division being renamed as Division Two. In addition, the league's three feeder leagues—the Combined Counties League, Essex Senior League and Spartan South Midlands League—ran in parallel with Division Two, and were able to feed directly into the regional Division Ones.

In 2004, The Football Association pushed through a major restructuring of the entire national non-league National League System, creating new regional divisions of the Football Conference feeding into the top, national, level. As a consequence of this restructuring, the Isthmian League was reduced back down to three divisions, and its boundaries were changed to remove the overlap with the Southern League.

In 2006, further reorganisation saw a reversion to two regional Division Ones and the disbandment of Division Two. This current plan calls for clubs based on the edges of the Isthmian League's territory to transfer to and from the Southern League as necessary to maintain numerical balance between the leagues. One team, Clapton, who were ever present in the Isthmian League since its foundation, were moved to the Essex Senior League for the 2006–07 season. Dulwich Hamlet, who had joined the league in 1907, became its longest serving member until their promotion to the National League South for the 2018–19 season.

In May 2017, The Football Association chose the Isthmian League to add a third regional division at Step 4 as part of further restructuring in the National League System, reducing all divisions at Step 4 to 20 teams. The new division started play in the 2018–19 season.