The history of postal service in St. Charles stretches back to the town's fledgling years. Appointed under President Van Buren, Horace Bancroft served as the first postmaster in 1837. Bancroft traveled by horseback to Elgin to gather mail from the post office there. On returning to St. Charles, he would distribute the mail from his office and dry goods store on the east side. During this early period, mail came only once a week from Naperville. By the 1840s, mail came two to three times per week via stagecoach. Since its establishment in the late 1830s, the post office has moved to several different locations. In the 1880s James Doherty built a frame building on West Main Street, and the post office was, for the first time, located in a building utilized solely for this purpose.
With the construction of the building at 108 S. 2nd Street in 1937, the post office remained in a single location for nearly 50 years. Built as a result of the PWA (Public Works Administration), the building was built on land donated by Mrs. E.J. Baker. Frazier and Raftery, architects from Geneva, designed the Classical Revival structure. The original plan of the building, which was designed by federal employees, brought opposition from the town's mayor, Dr. Ival Langum, and two of St. Charles's most prominent citizens, E.J. Baker and Lester Norris. Reportedly a nondescript, featureless box, these men believed that its architecture would greatly contrast with that of the surrounding buildings. Mayor Langum, Mr. Baker, and Mr. Norris went to Washington D.C. to protest the construction of such a building. Their objections were acknowledged and the post office was constructed of Bedford stone with a copper roof.

The post office vacated the site in 1985. Since this time Doc Morgan Inc., an awards and certificate manufacturer, has used the building. The new post office is located at 2600 Oak Street (south side of Rt. 64 just west of Randall Road), with a branch office at 616 East Main Street.