The 1984–85 season was the 70th season of the Isthmian League, which is an English football competition featuring semi-professional and amateur clubs from London, East and South East England.
The league consisted of three divisions. After 19 clubs, mainly from the Athenian League joined the league, Division Two was divided into two sections.
Also, it was the first Isthmian League season as an Alliance Premier League feeder league. Sutton United were champions, winning their second Isthmian League title. Wycombe Wanderers were promoted to the APL, as Sutton United and Worthing were ineligible for promotion. Epping Town resigned from the league midway through the season and folded.
| Competition: | Isthmian League Premier Division |
| Season: | 1984–85 |
| Winners: | Sutton United |
| Promoted: | Wycombe Wanderers |
| Relegated: | Harlow Town Leytonstone/Ilford |
| Matches: | 462 |
| Total Goals: | 1412 |
| Prevseason: | 1983–84 |
| Nextseason: | 1985–86 |
The Premier Division consisted of 22 clubs, including 20 clubs from the previous season and two new clubs, promoted from Division One:
| Competition: | Isthmian League Division One |
| Season: | 1984–85 |
| Winners: | Farnborough Town |
| Promoted: | Farnborough Town Kingstonian |
| Relegated: | Clapton Hertford Town Metropolitan Police Woking |
| Matches: | 462 |
| Total Goals: | 1496 |
| Prevseason: | 1983–84 |
| Nextseason: | 1985–86 |
Division One consisted of 22 clubs, including 18 clubs from the previous season and four new clubs:
Two clubs relegated from the Premier Division:
Two clubs promoted from Division Two:
| Competition: | Isthmian League Division Two North |
| Season: | 1984–85 |
| Winners: | Leyton-Wingate |
| Promoted: | Finchley Leyton-Wingate |
| Matches: | 380 |
| Total Goals: | 1176 |
| Prevseason: | 1983–84 |
| Nextseason: | 1985–86 |
Division Two North consisted of 21 clubs, including eight clubs transferred from Division Two, one team relegated from Division One and twelve new teams:
| Competition: | Isthmian League Division Two South |
| Season: | 1984–85 |
| Winners: | Grays Athletic |
| Promoted: | Grays Athletic Uxbridge |
| Matches: | 342 |
| Total Goals: | 1037 |
| Prevseason: | 1983–84 |
| Nextseason: | 1985–86 |
Division Two South consisted of 19 clubs, including eleven clubs transferred from Division Two, one team relegated from Division One and seven new clubs:
Clubs transferred from Division Two:
Club relegated from Division One:
Clubs joined from the Athenian League:
Plus: