In the years following the Civil War, the federal government´s headquarters building in St. Louis was the Old Custom House at Third and Olive Streets. It had become inadequate for its many uses. Erection of a larger replacement was authorized by Congress in 1869 and plans were prepared by government architect Alfred B. Mullett. The proposed location was criticized as being too far west from the business district of that time.

Excavation began in 1873 but a number of construction problems were encountered, including quick sand. Actual construction, therefore, did not begin until 1873. A prime reason for the site selection was its situation along the tunnel from Eads Bridge to the Union Depot. Access to the tunnel from the post office for direct dispatch of mail did not prove feasible because of seepage of smoke from the trains into the building, and the entry portal had to be sealed.

Construction proceeded slowly because of the great care required to make the structure completely fireproof and secure. It was built like a fortress with a 28-foot deep moat around the building and sliding iron shutters on its windows. These precautions were taken because of the presence of a subtreasury branch and as a guard against possible mob violence, a reminder from the recent Civil War.

No expense was spared on either the ornamental exterior or lavish interior of the great structure, which was finally completed in 1884 after 10 years of construction. Its cost of nearly $6 million made it the City´s most expensive building for some years to come.

In later years, various federal functions were relocated elsewhere, beginning with removal of the main post office to 18th and Clark in 1912. Two years later, the Weather Bureau vacated its roost in the cupola atop the Mansard dome for a perch on the roof of the Railway Exchange Building. Most of the federal courts and offices remained in the Old Post Office until 1935, when the new government building at 12th and Market was occupied.

The old structure has been vacant since 1975, when a postal branch was relocated across the street. For many years the old building was the object of much speculation regarding its fate, after a government announcement of possible demolition in 1964. This brought a concerted effort by preservationists to save it, with various proposals for its reuse. Its condition led to the passage of a bill by Congress authorizing private use of unused federal buildings.

Currently, plans are under consideration for the building´s use by Webster University as a downtown campus.