Mill Plain is an unincorporated area in Clark County, Washington, United States.

[citation needed] The Mill Plain area is located in southern Clark County.

According to the United States Census Bureau, in 2000 the CDP had a total area of 6.0 square miles (15.5 km2), all of it land.

There were 2,500 housing units at an average density of 417.9 per square mile (161.4/km2).

12.6% of all households were made up of individuals, and 3.5% had someone living alone who was 65 years of age or older.