The 2000 census lists the population of the city as 5,423.
According to the U.S. Census Bureau, the city has a total area of 11.1 square miles (28.8 km²), of which, 6.4 square miles (16.6 km²) of it is land and 4.7 square miles (12.2 km²) of it (42.23%) is water.
There were 2,035 households out of which 35.2% had children under the age of 18 living with them, 67.1% were married couples living together, 6.0% had a female householder with no husband present, and 25.4% were non-families. 22.9% of all households were made up of individuals and 14.5% had someone living alone who was 65 years of age or older. The average household size was 2.61 and the average family size was 3.10.
In the city the population was spread out with 26.2% under the age of 18, 5.2% from 18 to 24, 26.2% from 25 to 44, 24.7% from 45 to 64, and 17.7% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 90.8 males. For every 100 females age 18 and over, there were 88.6 males.
The median income for a household in the city was $56,699, and the median income for a family was $67,844. Males had a median income of $50,240 versus $30,273 for females. The per capita income for the city was $27,081. About 2.7% of families and 2.8% of the population were below the poverty line, including 2.8% of those under age 18 and 6.1% of those age 65 or over.
During the American Civil War, Spanish Fort was heavily fortified as an eastern defense to the city of Mobile. Fort Huger, Fort (Battery) Tracey, Fort (Battery) McDermott, Fort Alexis, Red Fort, and Old Spanish Fort were all part of the Mobile defenses in what is now Spanish Fort. After the Union victory in the Battle of Mobile Bay, Mobile nevertheless remained in Confederate hands. Union forces embarked on a land campaign in early 1865 to take Mobile from the east. Spanish Fort was the site of the Battle of Spanish Fort in the Mobile Campaign of the American Civil War. Its fall allowed Union forces to concentrate on Fort Blakeley to the north, and hence destroy the last organized resistance to northern occupation east of the Mississippi River. The falls of Spanish Fort and Fort Blakeley permitted Union troops to subsequently enter Mobile unopposed after the conclusion of the Civil War.
Two elementary schools, a middle school, and a high school serve Spanish Fort.