As of the census of 2000, there were 9,509 people, 4,063 households, and 2,372 families residing in the city. The population density was 1,657.4 people per square mile (639.6/km²). There were 4,463 housing units at an average density of 777.9/sq mi (300.2/km²). The racial makeup of the city was 94.35% White, 0.99% African American, 0.16% Native American, 2.53% Asian, 0.03% Pacific Islander, 0.73% from other races, and 1.21% from two or more races. Hispanic or Latino of any race were 2.64% of the population.
There were 4,063 households out of which 30.0% had children under the age of 18 living with them, 45.2% were married couples living together, 10.1% had a female householder with no husband present, and 41.6% were non-families. 35.9% of all households were made up of individuals and 12.0% had someone living alone who was 65 years of age or older. The average household size was 2.22 and the average family size was 2.90.
In the city the population was spread out with 23.7% under the age of 18, 8.7% from 18 to 24, 23.1% from 25 to 44, 30.9% from 45 to 64, and 13.6% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 92.7 males. For every 100 females age 18 and over, there were 89.1 males.
The median income for a household in the city was $31,202, and the median income for a family was $46,138. Males had a median income of $34,750 versus $24,830 for females. The per capita income for the city was $19,673. About 10.1% of families and 14.5% of the population were below the poverty line, including 17.7% of those under age 18 and 9.9% of those age 65 or over.
The nearest international commercial airport is in Cedar Rapids, approximately 90 miles (145 km) to the north. Bus service to Fairfield is provided by Greyhound Lines affiliates Jefferson Lines and Burlington Trailways.
Like many towns in the region, Fairfield was sited in the late 1830s along the route of one of the developing rail systems. Fairfield used to be served by Amtrak, the national passenger rail service. Amtrak carries passengers west-east on the California Zephyr, with passenger stations in Mount Pleasant, Iowa (25 miles to the east) and Ottumwa, Iowa (25 miles to the west).
The Rock Island Line also passed through Fairfield, but closed in the late 1970s. The old steel trestle has been removed from its crossing, and the walking trail that will some day circumnavigate the town uses part of the old roadbed.
Evidence of other long forgotten rail lines can be found in the woods around town. A section of narrow gauge roadbed can be found cutting its way through Whitham Woods, a park at the west edge of town.