The 1920–21 season was the 12th in the history of the Isthmian League, an English football competition.
Ilford were champions, winning their second Isthmian League title.
| Competition: | Isthmian League |
| Season: | 1920–21 |
| Winners: | Ilford |
| Matches: | 132 |
| Total Goals: | 489 |
| Prevseason: | 1919–20 |
| Nextseason: | 1921–22 |