Duncan township
- County:
- Mercer County
- County Seat:
- No
- Area (mi²):
- 36.636
- State:
- Illinois
Duncan township is located in Mercer County, Illinois. Duncan township has a 2026 population of 257. Duncan township is currently declining at a rate of -0.77% annually and its population has decreased by -1.91% since the most recent census, which recorded a population of 262 in 2020.
The median age in Duncan township is 62.4 years: 48.1 years for males, and 64.4 years for females. For every 100 females there are 115.0 males.