Isle is a city in Mille Lacs County, Minnesota, United States. The population was 803 at the 2020 census, up from 751 in 2010. A post …Isle is a city in Mille Lacs County, Minnesota, United States. The population was 803 at the 2020 census, up from 751 in 2010. A post office called Isle has been in operation since 1896. The city was named for the island near its harbor on Mille Lacs Lake.