Augusta is a city in St. Charles County, Missouri, United States. The population was 270 at the 2020 census, up from 253 at the 2010 cen…Augusta is a city in St. Charles County, Missouri, United States. The population was 270 at the 2020 census, up from 253 at the 2010 census. The city has wineries, antique shops, restaurants, B&B's, a wood shop, a glass studio, massage therapy, a historic museum, and The Augusta Brewery.