New Boston is a city in Mercer County, Illinois, United States on the Mississippi River. The population was 613 at the 2020 census, down …New Boston is a city in Mercer County, Illinois, United States on the Mississippi River. The population was 613 at the 2020 census, down from 683 in 2010. New Boston was settled in the 1830s by "Yankee" migrants from Massachusetts.