Willowbrook is a city in Reno County, Kansas, United States. As of the 2020 census, the population of the city was 71. It is a northwestern s…Willowbrook is a city in Reno County, Kansas, United States. As of the 2020 census, the population of the city was 71. It is a northwestern suburb of Hutchinson. According to the United States Census Bureau, the city has a total area of 0.31 square miles, all land.