Willoughby is a city in Lake County, Ohio, United States. It is a suburb of Cleveland, Ohio. The population was 22,621 at the 2000 census. Willoughby is served by a branch of the Willoughby-Eastlake Public Library. According to the United States Census Bureau, the city has a total area of 10.2 square miles (26 km2), of which, 10.2 square miles (26 km2) of it is land and 0.1 square miles (0.26 km2) of it (0.59%) is water.