Although Newcastle was not incorporated until 1994, it played a vital role in opening up the Seattle region for expansion. In many ways, it was the poster child for development in the Pacific Northwest in the late 1800s—rail, coal, and timber were the key themes of this time, and Newcastle was at the nexus of all of them. Newcastle's coal mine and railroad helped transform Seattle into a major port.
In 1872, between 75-100 tons of coal was produced each day in Newcastle; by the time the mine closed for the last time in 1963, more than 13 million tons had been extracted. Its legacy remains ever-present in local place names such as Coal Creek Parkway, and China Creek Golf Course. An active, well-informed, and passionate group of citizens remain committed to sharing Newcastle's history with visitors and residents. The city celebrates Newcastle Days at a community festival each September.
Based on per capita income Newcastle ranks 23rd of 522 areas in the state of Washington to be ranked.
As of the census of 2000, there were 7,737 people, 3,028 households, and 2,189 families residing in the city. The population density was 1,731.5 people per square mile (668.3/km²). There were 3,117 housing units at an average density of 697.6/sq mi (269.2/km²). The racial makeup of the city was 75.05% White, 1.62% African American, 0.45% Native American, 18.25% Asian, 0.25% Pacific Islander, 1.36% from other races, and 3.02% from two or more races. Hispanic or Latino of any race were 2.88% of the population.
There were 3,028 households out of which 34.3% had children under the age of 18 living with them, 63.7% were married couples living together, 5.8% had a female householder with no husband present, and 27.7% were non-families. 20.5% of all households were made up of individuals and 2.8% had someone living alone who was 65 years of age or older. The average household size was 2.55 and the average family size was 2.98.
In the city the population was spread out with 23.4% under the age of 18, 7.2% from 18 to 24, 37.9% from 25 to 44, 24.9% from 45 to 64, and 6.6% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 100.0 males. For every 100 females age 18 and over, there were 97.9 males.
The median income for a household in the city was $80,320, and the median income for a family was $91,381. Males had a median income of $60,639 versus $41,868 for females. The per capita income for the city was $35,057. About 1.8% of families and 2.0% of the population were below the poverty line, including 1.1% of those under age 18 and 6.6% of those age 65 or over.
Although the present-day African American population of Newcastle is small, in the 1880s when it was a mining center, Newcastle had the Puget Sound area's largest African American population, greater even than that of Seattle.