Mentor is a city in Polk County, Minnesota, United States. It is part of the Grand Forks-ND-MN Metropolitan Statistical Area. The population wa…Mentor is a city in Polk County, Minnesota, United States. It is part of the Grand Forks-ND-MN Metropolitan Statistical Area. The population was 104 at the 2020 census. The city is near Maple Lake. A post office called Mentor has been in operation since 1882. The city was named after Mentor, Ohio.