As is the case with many cities dotting America's 21st-century landscape, Portland grew to prominence during the last half of the century, as seen by this table.
Portland, Oregon: Population, 1850-1900
For ocean port cities like San Fransisco, Portland, and Seattle, not only did trains provide a link to the nation, shipping provided a link to the world, hence the need for a customs house for which government officials could process goods imported and exported to and from the U.S. through Portland.
In 1875, the U.S. Customs Service first established an office within Portland's new U.S. Post Office. However, as the city grew, and trade to foreign nations increased, the U.S. Customs Service outgrew that original space. Hence, in 1898, construction began on the present U.S. Custom House, reaching completion in 1901.
It functioned as a customs house through the 1960s. In 1938, continued growth inspired the construction of fourth floors, which were added to the building's east and west wings. And then, in 1968, the history of the U.S. Customs Service came full-circle; the moved into the Old Post Office Building. The North Pacific Division of the U.S. Corps of Engineers moved into the building left by the Customs Service.