As of the censusGR2 of 2000, there were 19,973 people, 8,460 households, and 5,422 families residing in the city. The population density was 1,036.5/km² (2,683.0/mi²). There were 9,007 housing units at an average density of 467.4/km² (1,209.9/mi²).
There were 8,460 households out of which 26.0% had children under the age of 18 living with them, 47.3% were married couples living together, 12.6% had a female householder with no husband present, and 35.9% were non-families. 28.5% of all households were made up of individuals and 9.6% had someone living alone who was 65 years of age or older. The average household size was 2.33 and the average family size was 2.85.
In the city the population was spread out with 22.4% under the age of 18, 8.9% from 18 to 24, 29.3% from 25 to 44, 23.1% from 45 to 64, and 16.3% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 96.2 males. For every 100 females age 18 and over, there were 94.5 males.
The median income for a household in the city was $40,153, and the median income for a family was $45,791. Males had a median income of $29,709 versus $21,641 for females. The per capita income for the city was $21,085. About 7.3% of families and 9.9% of the population were below the poverty line, including 16.2% of those under age 18 and 4.1% of those age 65 or over.
The city of Fort Walton Beach is surrounded by a larger urbanized area which includes other incorporated and unincorporated areas. The population for the "Fort Walton Beach County Subdivision" according to the 2000 Census is 84,337.