Umatilla's earliest importance was as a trade and distribution center on the Columbia River, during the gold rush of the 1860s and 1870s in eastern Oregon. It remained a major commercial center until the 1880s when it withered before the Oregon Railway and Navigation Company.
The Umatilla Chemical Depot opened in 1941, to gear up for World War II. The depot's mission was to store and maintain a variety of military items, from blankets to ammunition. The depot took on its chemical weapons storage mission in 1962. From 1990 to 1994 the facility reorganized in preparation for eventual closure, shipping all conventional ammunition and supplies to other installations. Today, the chemical weapons are the only items still stored at the depot.
According to the United States Census Bureau, the city has a total area of 3.7 square miles (9.7 km²), of which, 3.5 square miles (9.1 km²) of it is land and 0.2 square miles (0.5 km²) of it (5.63%) is water.
There were 1,364 households out of which 45.7% had children under the age of 18 living with them, 58.2% were married couples living together, 13.0% had a female householder with no husband present, and 22.1% were non-families. 16.9% of all households were made up of individuals and 5.1% had someone living alone who was 65 years of age or older. The average household size was 3.14 and the average family size was 3.51.
In the city the population was spread out with 31.0% under the age of 18, 12.1% from 18 to 24, 34.7% from 25 to 44, 16.5% from 45 to 64, and 5.7% who were 65 years of age or older. The median age was 29 years. For every 100 females there were 137.8 males. For every 100 females age 18 and over, there were 157.3 males.
The median income for a household in the city was $33,844, and the median income for a family was $32,969. Males had a median income of $28,500 versus $20,337 for females. The per capita income for the city was $11,469. About 15.6% of families and 19.4% of the population were below the poverty line, including 24.2% of those under age 18 and 21.1% of those age 65 or over.