La stagione 2020-21 è stata la 105ª stagione della Isthmian League, una competizione calcistica inglese con squadre semiprofessionali e dilettanti di Londra, Inghilterra orientale e sud-orientale. La lega gestisce quattro divisioni, la Premier Division alla Fase 3 e tre divisioni, Nord, Centro-Sud e Sud-Est alla Fase 4 del Sistema Lega Nazionale. Questa è stata la terza stagione a consistere in quattro divisioni dopo che la lega ha riorganizzato l'ex South Division nelle nuove divisioni South Central e South East.
Le assegnazioni per i passaggi 3 e 4 per la stagione 2020-21 sono state annunciate dalla FA il 21 luglio. Non sono state apportate modifiche alle costituzioni di nessuna delle divisioni all'interno della Isthmian League.
Premier Division Squadre partecipanti SquadraCittàStadioCapienza Bishop's Stortford Woodside Park 4525 Bognor Regis Nyewood Lane 4500 Bowers & Pitsea Pitsea Len Salmon Stadium 2000 Brightlingsea Regent Brightlingsea North Road 2000 Carshalton War Memorial Sports Ground 5000 Cheshunt Theobalds Lane 3000 Tolworth King George's Field 2700 St Mary Cray Hayes Lane 6000 Corringham Rookery Hill 4000 Enfield Town Enfield Queen Elizabeth II Stadium 2500 Folkestone Cheriton Road 4000 Tottenham Coles Park 2500 Hornchurch Hornchurch Stadium 3500 Horsham The Camping World Community Stadium 1300 Kingston upon Thames King George's Field 2700 Leatherhead Fetcham Grove 3400 Lewes The Dripping Pan 3000 Margate Hartsdown Park 3000 Merstham Merstham Moatside 2000 Potters Bar Parkfield 2000 Wingate & Finchley Finchley The Maurice Rebak Stadium 1500 Worthing Woodside Road 4000
Classifica finale Squadra1.2187012210+12 2.20116231314-13. Enfield Town18106041517-24.1685121410+45.1575022110+116.1595041518-37.146420135+88. 14104241712+59. Horsham14104241915+410.1394141313011.1284041313012.129333815-713. Bowers & Pitsea 105311135+814.107412126+615.1093151311+216. Wingate & Finchley1083141817-1 17.109315913-418.88224815-719. Brightlingsea Regent8112271120-9 20.69135613-721.591261021-11 22. Merstham48116818-10
North Division Squadre partecipanti SquadraCittàStadioCapienza ACF Sudbury Sudbury King's Marsh 2500 Aveley Parkside 3500 Basildon Gardiners Close 2000 Brentwood The Brentwood Centre Arena 1800 Bury St Edmunds Ram Meadow 3500 Cambridge Bridge Road 2000 Canvey Island Park Lane 4500 Coggeshall Town Coggeshall West Street 2000 Dereham Aldiss Park 3000 Felixstowe & Walton United Felixstowe Dellwood Avenue 2000 Grays Parkside 3500 Great Wakering Rovers Great Wakering Burroughs Park 2500 Heybridge Swifts Heybridge Scraley Road 3000 Histon and Impington Bridge Road 4300 Hullbridge Sports Hullbridge Lower Road 1500 Maldon & Tiptree Maldon Wallace Binder Ground 2000 Romford The Brentwood Centre Arena 3500 Soham Julius Martin Lane 2000 Tilbury Chadfields 4000 Witham Town Witham Spa Road 2500
Classifica finale Squadra1.168512149+52. AFC Sudbury1283321514+13. Maldon & Tiptree 11532093+64. Bury Town10431092+75. Heybridge Swifts 108314109+1 6.106312109+1 7. Soham Town Rangers107313770 8. Grays Athletic10631256-19. 9630389-110.98233710-311. Coggeshall Town8622276+1 12. Dereham Town8722377013. Felixstowe & Walton United8522177014. Great Wakering Rovers 872231114-315. Canvey Island6513185+316. Hullbridge Sports65203119+2 17.6420233018. Brentwood Town6513156-119. Basildon Town56123119+220. Witham Town38107727-20
South Central Division Squadre partecipanti SquadraCittàStadioCapienza Ashford Town Stanwell Robert Parker Stadium 2550 Barking Mayesbrook Park 2500 Bedfont Bedfont Recreation Ground 3000 Bracknell The Brentwood Centre Arena 2500 Chalfont St Peter Mill Meadow 1500 Chertsey Town Chertsey Alwyns Lane 2500 Chipstead Chipstead High Road 2000 FC Romania Cheshunt Cheshunt Stadium 3000 Hanwell Town Perivale Reynolds Field 3000 Harlow The Harlow Arena 3500 Hertford Hertingfordbury Park 6500 Marlow Marlow Alfred Davis Memorial Ground 3000 Northwood Northwood Park 3075 South Park Reigate King George's Field 2000 Staines-upon-Thames Wheatsheaf Park 3000 Tooting & Mitcham United Mitcham Imperial Fields 3500 Uxbridge West Drayton Honeycroft 3770 Waltham Abbey Waltham Abbey Capershotts 3500 Ware Wodson Park 3300 Westfield Woking Woking Park 1000
Classifica finale Squadra1. Waltham Abbey168512139+42.168512171703. Ware1575022010+104. Tooting & Mitcham United157502123+95. Bracknell Town135410145+96. Chertsey Town 137412149+57. Hanwell Town1384131512+38. Ashford Town1384138809. Hertford Town1274031010010. Marlow117322168+811. Westfield1073131511+412.10731399013. Chipstead1083141219-714. Bedfont Sports96303108+215. Chalfont St Peter98305610-416. FC Romania67205814-617. Uxbridge 56123511-618. South Park571241220-819.481161912-720.38107617-11
South East Division Cambiamenti di squadra dalla scorsa stagione La South East Division comprende 19 delle 20 squadre che hanno gareggiato nella competizione interrotta la stagione precedente. Guernsey si è ritirato da questa stagione a causa di restrizioni di viaggio.
Squadre partecipanti SquadraCittàStadioCapienza Ashford The Homelands 3200 Burgess Hill Town Burgess Hill Leylands Park 2500 Chichester City Chichester Oaklands Park 2000 Cray Valley Paper Mills Eltham Badgers Sports Ground 1000 East Grinstead Town East Grinstead East Court 1500 Faversham Town Faversham Salters Lane 2000 Hastings The Pilot Field 4050 Haywards Heath Town Haywards Heath Hanbury Park 2000 Herne Bay Herne Bay Winch's Field 4000 Hythe Reachfields Stadium 3000 Phoenix Sports Barnehurst Victory Road 2000 Ramsgate Southwood Stadium 2500 Sevenoaks Town Sevenoaks Greatness Park 1000 Sittingbourne Woodstock Park 3000 Three Bridges Crawley Jubilee Field 1500 VCD Athletic Crayford Oakwood 1400 Brighton The Enclosed Ground 3126 Whitstable Town Whitstable The Belmont Ground 3000 Whyteleafe Church Road 2000
Classifica finale Squadra1.177520133+102. VCD Athletic168512187+113. East Grinstead Town146420166+104.136411157+85. Sevenoaks Town1383411410+46. Faversham Town12633083+57. Whitstable Town1193241220-88.1063121412+29.762131010010. Herne Bay7621399011.76213911-212. Cray Valley Paper Mills6513166013. Three Bridges662041115-414. Chichester City65203711-415. Phoenix Sports67205716-916.5612479-217. Haywards Heath Town571241119-818.45113812-419. Burgess Hill Town47115514-9
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Isthmian League