According to the United States Census Bureau, the city has a total area of 3.5square miles (9.1Square kilometers), all of it land. Buffalo is a city in Johnson County, Wyoming, United States. The population was 3,900 at the 2000 census. It is the county seat of Johnson County. In recent years, the town has boomed economically due to methane production from the coal bed methane extraction method used in the Powder River Basin and surrounding areas. As of the census of 2000, there were 3,900 people, 1,718 households, and 1,042 families residing in the city. The population density was 1,104.8 people per square mile (426.6Square kilometers). There were 1,842 housing units at an average density of 521.8per square mile (201.5Square kilometers).