Mountlake Terrace, WA has a population of 20,747 and is the 2,088th largest city in the United States. The population density is 5,091 per sq mi which is 5166% higher than the Washington average and 5519% higher than the national average. The median age in Mountlake Terrace is 37 which is approximately 2% lower than the Washington average of 38. In Mountlake Terrace, 52% of the population over 15 years of age are married, 77% speak English and 6% speak Spanish. 48% of Mountlake Terrace residents were born in Washington, 30% were born out of state, 3% were born outside of the United States and 19% were foreign born.