Brown City is a city in Lapeer and Sanilac counties in the U.S. state of Michigan. The population was 1,325 at the 2010 census. The city is l…Brown City is a city in Lapeer and Sanilac counties in the U.S. state of Michigan. The population was 1,325 at the 2010 census. The city is located almost exclusively in Sanilac County, with only a small portion of the city extending into Lapeer County.