Lengby is a city in Polk County, Minnesota, United States. It is part of the Grand Forks ND-MN Metropolitan Statistical Area. The population was 92 at the 2020 census. A post office called Lengby has been in operation since 1898. Lengby had a depot on the railroad. In 1980, Jean Hilliard survived being frozen for six hours in subzero temperatures; her story was later featured on Unsolved Mysteries.