is a city in southwestern Warren County, Ohio, United States. As of the 2000 census, Mason's population was 22,016. It was the fastest-growing and most populous city in the county. Until February 1997, it was part of Deerfield Township.
On June 1, 1803, Revolutionary War veteran William Mason paid $1,700 at auction to purchase 640 acres of land in what is now downtown Mason. In 1815, he platted 16 lots on this land and named the village "Palmira." In 1832, 2 years after the death of William Mason and according to his will, over 40 more lots were platted on the north, south, and west of Palmira. When the plat was officially recorded, the name of the village was listed as "Palmyra."
In 1835, a petition was sent to the federal post office to correct the name of the town. It had been listed as Kirkwood, possibly an error because the postmaster at the time was William Kirkwood. When village officials were informed that there was another Palmyra in Ohio, the name was officially changed to "Mason." Mason remained a small farming community for another 125 years. In 1970, a year before the town was incorporated to become a city, there were fewer than 5,700 residents.