Atlantic Ocean shoreline. Margate City is a city in Atlantic County, New Jersey, United States. As of the 2010 United States Census, the city's population was 6,354, reflecting a decline of 1,839 (-22.4%) from the 8,193 counted in the 2000 Census, which had in turn declined by 238 (-2.8%) from the 8,431 counted in the 1990 Census.