According to the United States Census Bureau, the city has a total area of 7.5 square miles (19.3 km²). None of the area is covered with water.
As of the census of 2000, there were 7,596 people, 2,522 households, and 2,042 families residing in the city. However, by 2003, that number had nearly doubled to 13,846. In 2000, The population density was 1,016.2 people per square mile (392.6/km²). There were 2,612 housing units at an average density of 349.4 per square mile (135.0/km²). The racial makeup of the city was 86.22% White, 2.91% African American, 0.92% Native American, 0.51% Asian, 0.13% Pacific Islander, 6.94% from other races, and 2.37% from two or more races. Hispanic or Latino of any race were 15.94% of the population.
51.7% of households have children under the age of 18 living with them, 65.5% were married couples living together, 10.5% had a female householder with no husband present, and 19.0% were non-families. 14.8% of all households were made up of individuals and 3.6% had someone living alone who was 65 years of age or older. The average household size was 3.01 and the average family size was 3.33.
In the city the population was spread out with 33.5% under the age of 18, 7.6% from 18 to 24, 38.6% from 25 to 44, 15.7% from 45 to 64, and 4.6% who were 65 years of age or older. The median age was 30 years. For every 100 females there were 102.3 males. For every 100 females age 18 and over, there were 96.0 males.
The median income for a household in the city was $53,504, and the median income for a family was $55,051. Males had a median income of $36,021 versus $27,413 for females. The per capita income for the city was $20,263. About 2.5% of families and 4.2% of the population were below the poverty line, including 3.6% of those under age 18 and 3.0% of those age 65 or over.
On August 15 and September 5, 2011, Leander was the scene of two destructive wildfires that swept through two central neighborhoods, together burning 330 acres (134 ha) and destroying 26 homes. See 2011 Texas wildfires.