Markham is a city and a south suburb of Chicago in Cook County, Illinois, United States. The population was 11,661 at the 2020 census. It is cl…Markham is a city and a south suburb of Chicago in Cook County, Illinois, United States. The population was 11,661 at the 2020 census. It is claimed this area was beach 10,000 years ago. After countless ages of geologic swamps, marshes and sloughs, the prairies dominated the landscape with groves of trees, flowers, and wildlife in abundance.