Beacon is a city in Mahaska County, Iowa, United States. The population was 445 at the 2020 census. Coal miners in Beacon joined the Uniteā¦Beacon is a city in Mahaska County, Iowa, United States. The population was 445 at the 2020 census. Coal miners in Beacon joined the United Mine Workers union in 1894, organizing local 178. By 1902, this had 100 members.