St. Helens is the county seat of Columbia County, Oregon. It was founded by Captain H. M. Knighton, a native of New England, in 1845 as "Plymouth". The name was changed to St. Helens in the latter part of 1850 for its view of Mount St. Helens some 39 miles (63 km) away in Washington. The population was 12,883 at the 2010 census.

St. Helens was established as a river port on the Columbia River in the 1840s. In 1853, the Pacific Mail Steamship Company tried to make the city their only stop on the Columbia River. Portland's merchants boycotted this effort, and the San Francisco steamship Peytona helped break the impasse.

St. Helens was incorporated as a city in 1889.

The Lewis and Clark Expedition passed through and camped in the area that is now St. Helens on the night of November 5, 1805 while on their way to the Pacific Ocean. While here the party encountered Native Americans and Clark observed "low rockey clifts".

U.S. Route 30 passes through the city.

According to the United States Census Bureau, the city has a total area of 5.51 square miles (14.27 km²), of which, 4.53 square miles (11.73 km²) is land and 0.98 square miles (2.54 km²) is water.

According to the Köppen climate classification system, St. Helens has a warm-summer Mediterranean climate (Csb).

Columbia Heights is a formerly separate populated place that is within the city limits of St. Helens.

As of the census of 2010, there were 12,883 people, 4,847 households, and 3,243 families residing in the city. The population density was 2,843.9 inhabitants per square mile (1,098.0/km²). There were 5,154 housing units at an average density of 1,137.7 per square mile (439.3/km²). The racial makeup of the city was 90.3% White, 0.6% African American, 1.6% Native American, 1.3% Asian, 0.3% Pacific Islander, 1.3% from other races, and 4.5% from two or more races. Hispanic or Latino of any race were 6.1% of the population.

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