| | Mason City, IA | Ottumwa, IA | United States |
| Population | 27,385 | 25,379 | 329,725,481 |
| Female Population | 51.2% | 49.4% | 50.5% |
| Male Population | 48.8% | 50.6% | 49.5% |
| Median Age | 42.5 | 37.7 | 38.4 |
| space3 | | | |
| Population - 2010 | 28,042 | 25,061 | 309,327,143 |
| Population - 2000 | 29,172 | 24,998 | 285,036,114 |
| Population - 1990 | 29,040 | 24,488 | 251,960,433 |
| Pop. 2000 to Now | -8.5% | -2.5% | 15.6% |
| Pop. 1990 to Now | -8.1% | -0.5% | 30.8% |
| space4 | | | |
| Population Density | 985.3 | 1,600.5 | 93.4 |
| Land Area | 27.8 | 15.9 | 3,531,905.4 |
| Water Area | 0.3 | 0.7 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Mason City, IA | Ottumwa, IA | United States |
| Age 0 to 4 | 5.3% | 6.5% | 5.9% |
| Age 5 to 9 | 5.1% | 5.8% | 6.1% |
| Age 10 to 14 | 7.1% | 6.7% | 6.6% |
| Age 15 to 17 | 3.2% | 4.2% | 3.9% |
| Age 18 to 20 | 3.8% | 4.4% | 4.0% |
| Age 21 to 24 | 4.4% | 5.4% | 5.2% |
| Age 25 to 34 | 12.8% | 13.9% | 13.8% |
| Age 35 to 44 | 11.1% | 11.3% | 12.9% |
| Age 45 to 54 | 11.8% | 13.1% | 12.6% |
| Age 55 to 59 | 6.4% | 5.9% | 6.7% |
| Age 60 to 64 | 8.2% | 7.4% | 6.3% |
| Age 65 to 74 | 11.7% | 8.4% | 9.6% |
| Age 75 to 84 | 5.9% | 4.4% | 4.5% |
| Age 85 and over | 3.3% | 2.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | Mason City, IA | Ottumwa, IA | United States |
| White | 87.5% | 74.8% | 59.4% |
| Black | 2.6% | 4.5% | 12.2% |
| Asian | 0.8% | 1.7% | 5.6% |
| Native American | 0.4% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.3% | 1.6% | 0.2% |
| Other | 0.1% | 0.0% | 0.4% |
| Two or More Races | 2.2% | 2.6% | 3.2% |
| Hispanic | 6.1% | 14.9% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Mason City, IA | Ottumwa, IA | United States |
| Married Population | 51.9% | 45.3% | 49.8% |
| Currently Married | 50.7% | 43.9% | 48.0% |
| Married but Separated | 1.2% | 1.4% | 1.8% |
| Single Population | 48.1% | 54.7% | 50.2% |
| Never Married | 29.1% | 34.1% | 33.8% |
| Divorced | 12.0% | 13.8% | 10.8% |
| Widowed | 7.0% | 6.9% | 5.6% |
| space5 | | | |
| Household Size | 2.09 | 2.40 | 2.60 |
| Households | 12,744 | 10,279 | 124,010,992 |
| Family Households | 7,264 | 6,482 | 80,755,759 |
| Married couple, w/children | 23.6% | 24.3% | 30.5% |
| Married couple, no children | 51.4% | 41.2% | 42.9% |
| Non Family Households | 5,480 | 3,797 | 43,255,233 |
| Single householder, w/children | 16.5% | 25.9% | 15.8% |
| Single householder, no children | 8.4% | 8.5% | 10.8% |