City in Warren County, Illinois · 4.2 sq mi · 2,004 people per sq mi
8,502 people live in Monmouth. The typical household earns $61,138, and the median resident is 34.1 years old.
2026 projection
From 8,902 in 2020
Poverty rate 17.71%
Men 33.2 · Women 36.2
1850–2026
8,502
2026 projection
4.5% since 2020 (decrease)
Source: U.S. Census Bureau decennial counts and annual estimates, including 2010–2020 intercensal estimates. 2026 is a projection. Values between recorded years are interpolated.
Use the left and right arrow keys to inspect population observations.
1850–2026
8,502
2026 projection
4.5% since 2020 (decrease)
Source: U.S. Census Bureau decennial counts and annual estimates, including 2010–2020 intercensal estimates. 2026 is a projection. Values between recorded years are interpolated.
Use the left and right arrow keys to inspect population observations.
| 2026 | 8,502 | -97 | 1.13% |
|---|---|---|---|
| 2025 | 8,599 | +64 | 0.75% |
| 2024 | 8,535 | +5 | 0.06% |
| 2023 | 8,530 | -58 | 0.68% |
| 2022 | 8,588 | -227 | 2.58% |
| 2021 | 8,815 | -87 | 0.98% |
| 2020 | 8,902 | -47 | 0.53% |
| 2019 | 8,949 | -75 | 0.83% |
| 2018 | 9,024 | -76 | 0.84% |
| 2017 | 9,100 | -74 | 0.81% |
Showing 10 of 33 years.
The 2020 row compares the April 1 Census count with the July 1, 2019 estimate (about nine months). Its Growth and Annual change therefore do not represent a full year.
Monmouth has a population change about the same as Warren County, and lower than Illinois and the U.S.
Change from July 1, 2020 to July 1, 2025 population estimates—not annualized growth or site projections.
Source: U.S. Census Bureau. U.S. totals are summed from the 50 states and D.C.; unavailable comparisons are omitted.
U.S. Census Bureau, 2024 ACS 5-year estimates · Table S0101
Each square is about 1% of residents (roughly 86 people).
Hispanic or Latino residents can be of any race. All other groups exclude Hispanic residents.
U.S. Census Bureau, 2024 ACS 5-year estimates · Table B03002
Half of households earn more, half earn less
Typical yearly pay for a worker 25 or older.
About 1 in 6 residents
36% of residents age 15 and older are married.
| Status | Overall | Male | Female |
|---|---|---|---|
| Married | 36% | 40% | 31% |
| Never married | 46% | 48% | 44% |
| Divorced | 10% | 10% | 11% |
| Widowed | 7% | 2% | 10% |
| Separated | 2% | <1% | 4% |
U.S. Census Bureau, 2024 ACS 5-year estimates · Table S1201
14 sources