There were 6,563 households of which 40.3% had children under the age of 18 living with them, 50.7% were married couples living together, 12.6% had a female householder with no husband present, 5.6% had a male householder with no wife present, and 31.1% were non-families. 24.0% of all households were made up of individuals and 10.2% had someone living alone who was 65 years of age or older. The average household size was 2.70 and the average family size was 3.21.
The median age in the city was 34.3 years. 28.3% of residents were under the age of 18; 8.7% were between the ages of 18 and 24; 29.2% were from 25 to 44; 22.4% were from 45 to 64; and 11.3% were 65 years of age or older. The gender makeup of the city was 48.6% male and 51.4% female.
As of the census of 2000, there were 12,750 people, 4,281 households, and 3,095 families residing in the city. The population density was 1,548.4 people per square mile (598.2/km²). There were 4,516 housing units at an average density of 597.0 per square mile (230.6/km²). The racial makeup of the city was 90.01% White, 1.13% African American, 1.04% Native American, 2.20% Asian, 0.32% Pacific Islander, 2.47% from other races, and 2.83% from two or more races. Hispanic or Latino of any race were 5.83% of the population.
There were 4,281 households out of which 42.6% had children under the age of 18 living with them, 56.7% were married couples living together, 11.5% had a female householder with no husband present, and 27.7% were non-families. 22.7% of all households were made up of individuals and 9.3% had someone living alone who was 65 years of age or older. The average household size was 2.72 and the average family size was 3.19.
In the city the age distribution of the population shows 31.5% under the age of 18, 8.0% from 18 to 24, 32.6% from 25 to 44, 18.4% from 45 to 64, and 9.6% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 93.3 males. For every 100 females age 18 and over, there were 91.2 males.
The median income for a household in the city was $400,000, and the median income for a family was $51,941. Males had a median income of $41,517 versus $26,912 for females. The per capita income for the city was $19,146. About 5.8% of families and 7.2% of the population were below the poverty line, including 9.2% of those under the age of 18 and 10.4% of those age 65 and older.
From its founding, Arlington was dependent on timber harvesting and timber products. It was once dubbed the "Shingle Capital of the World" owing to the shingle mills, sawmills, and logging camps in the vicinity.
Historically, Arlington was a railroad town with several different rail connections. It was located on the Northern Pacific(NP) mainline that stretched from the Canadian border through Sedro-Woolley to Snohomish. Arlington also had a branch line running east to Darrington that primarily carried timber products. Both of these lines were abandoned by NP's successor Burlington Northern in the 1980s due to declining industry. While it is not as rail dependent as it once was, Arlington still maintains a rail connection with BNSF Railways via a spur line. Remnants of Arlington's railroad history can still be seen with the various trestles and signal equipment still in place. Snohomish County is in the process of converting the abandoned rail right of way into bike/hike trails.
On October 19, 1959 a Boeing 707-227 registration N7071 crashed on a Stillaguamish Riverbed northeast of town. It was the first of five Boeing 707s destined for delivery to Braniff International Airways. A Boeing test pilot and Braniff Captain were killed in an emergency landing after three of the aircraft's four engines were torn off during a training maneuver.
The largest planned development in Arlington, Gleneagle is a 1,037-unit neighborhood and HOA built in and around the Gleneagle Golf Course. Consisting of single-family homes and townhouses, Gleneagle was primarily built up from 1987 through 2002 and houses approximately 30% of Arlington's population.
Arlington Heights is a residential area situated east of Arlington on the triangular raised plateau bordered on the east by the Cascade Mountains, on the northwest by the North Fork of the Stillaguamish River, and on the southwest by the South Fork of the Stillaguamish River. In the Cascade foothills east of Arlington Heights is the Jim Creek Naval Radio Station, a very low frequency radio array for communication with submarines. It is largely decommissioned but still in use as a recreation facility by Navy personnel and families. Arlington Heights is outside the municipal boundaries of Arlington, but its residents have Arlington postal addresses and their children attend Arlington schools. Some areas in Arlington Heights have excellent views of Whitehorse and Three Fingers Mountains.