The Post Office was established as Reesville, Montgomery County on January 18, 1820. The name was changed to Upper Merion on February 28, 1829, and changed again to King of Prussia on April 5, 1837. It’s the only post office with this name in the world! There has never been an incorporated City of King of Prussia; the official name of the municipality is Upper Merion Township, which was established in 1713 and incorporated in 1789. The fire department uses King of Prussia, while the police department uses Upper Merion as their name.
(Source: Upper Merion Township The First 300 Years, p 8.)
E-History Fact published 2/27/2015 Upper Merion Township E-Newsletter