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. Winthrop is located at 42°28′21″N 91°44′9″W / 42.47250°N 91.73583°W.