Gardner is a city in Cass County, North Dakota, United States. The population was 129 at the 2020 census. The first settlement at Gardn…Gardner is a city in Cass County, North Dakota, United States. The population was 129 at the 2020 census. The first settlement at Gardner was made in 1880. A post office has been in operation at Gardner since 1881. Gardner was platted in 1882 when the railroad was extended to that point. Gardner was named in honor of Stephen Gardner, who owned the land where the city is located.