Greentop is a city in Adair and Schuyler counties, Missouri, United States. As of the 2020 census, its population was 388. Greentop is considere…Greentop is a city in Adair and Schuyler counties, Missouri, United States. As of the 2020 census, its population was 388. Greentop is considered one of the oldest communities in Schuyler County, with its first settlement in the early 1840s. However, the town layout was not documented until 1855. A U.S. Post Office was established in 1857, and the town was finally incorporated in February 1867.