Marengo is a city in McHenry County, Illinois, United States on the Kishwaukee River. It lies approximately 60 miles west northwest of C…Marengo is a city in McHenry County, Illinois, United States on the Kishwaukee River. It lies approximately 60 miles west northwest of Chicago and approximately 30 miles east of Rockford. Per the 2020 census, the population was 7,568.