Arnold is a city in northeastern Jefferson County, Missouri, situated near the confluence of the Meramec and Mississippi rivers. A suburb of St…Arnold is a city in northeastern Jefferson County, Missouri, situated near the confluence of the Meramec and Mississippi rivers. A suburb of St. Louis, the city lies within the Greater St. Louis metropolitan statistical area. The population was 20,858 at the 2020 United States census, which makes it the most populous community in the county.