Frankfort is a village in Will County and Cook County in the U.S. state of Illinois. It is a southern suburb of Chicago, and is approximately 28 miles south of the city. As of the 2020 census, the population was 20,296.Frankfort’s historic downtown area contains the Frankfort Grainery, Breidert Green, and a portion of the Old Plank Road Trail, a 22 mile-long paved recreation trail.