According to the United States Census Bureau, the city has a total area of 1.5 square miles (4.0 km²), of which, 1.4 square miles (3.6 km²) of it is land and 0.2 square miles (0.4 km²) of it (9.74%) is water.
There were 645 households out of which 30.5% had children under the age of 18 living with them, 57.8% were married couples living together, 10.2% had a female householder with no husband present, and 29.6% were non-families. 25.0% of all households were made up of individuals and 9.5% had someone living alone who was 65 years of age or older. The average household size was 2.47 and the average family size was 2.92.
In the city the population was spread out with 25.8% under the age of 18, 6.8% from 18 to 24, 27.1% from 25 to 44, 25.5% from 45 to 64, and 14.9% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 96.9 males. For every 100 females age 18 and over, there were 96.0 males.
The median income for a household in the city was $41,106, and the median income for a family was $51,000. Males had a median income of $41,411 versus $25,714 for females. The per capita income for the city was $18,759. About 7.4% of families and 11.3% of the population were below the poverty line, including 15.2% of those under age 18 and 9.2% of those age 65 or over.
In 2006, the Weyerhaueser pulp mill in Cosmopolis shut down, costing the local community 245 jobs. Previously, in late 2005, the Weyerhaueser large-log saw mill in Aberdeen closed, losing 95 jobs. Currently, there are plans for a new biodiesel plant and renewed hope that the industry will save the city.