washtenaw_county,_michigan | country | united_states <tsp> ann_arbor,_michigan | ispartof | washtenaw_county,_michigan <sep> washtenaw county ( wash-tə-naw) is a county located in the us state of michigan at the 2020 census, the population was 372,258 the county seat is ann arbor the county was authorized by legislation in 1822 and organized as a county in 1826 
ann arbor. michigan is part of washtenaw county. united states.washtenaw county michigan is a u.s. state