Fairmont is a city in and the county seat of Martin County, Minnesota, United States. The population was 10,487 at the 2020 census. Fairm…Fairmont is a city in and the county seat of Martin County, Minnesota, United States. The population was 10,487 at the 2020 census. Fairmont was platted in 1857. The city was so named on account of its elevated town site. A post office has been in operation at Fairmont since 1858. Fairmont was incorporated as a city on February 28, 1878, with the first city charter adopted in 1902.