Frankfort is a city in Marshall County, Kansas, United States. As of the 2020 census, the population of the city was 730. Frankfort was laid o…Frankfort is a city in Marshall County, Kansas, United States. As of the 2020 census, the population of the city was 730. Frankfort was laid out in 1867 when the Central Branch Union Pacific Railroad was built through the area. The nearby post office of Nottingham, established in 1857, moved roughly a half-mile northwest to meet the railroad. It was named for Frank Schmidt, a member of the town company. Frankfort was incorporated as a city of the third class in 1875.