Winthrop is a city in Buchanan County, Iowa, United States. The population was 823 at the time of the 2020 census. Winthrop was pla…Winthrop is a city in Buchanan County, Iowa, United States. The population was 823 at the time of the 2020 census. Winthrop was platted in 1857. According to the United States Census Bureau, the city has a total area of 0.84 square miles, all land.