In the post-pandemic era, cities are facing new demands and challenges. Both telecommuting and dense development require communities to provide more public space to meet people’s psychological and physical needs. The insufficient space currently left in the city calls for stock development. The Japanese public libraries affected by COVID-19 are part of this stock. We aimed to determine and propose strategies for using the external space of libraries to enhance the sustainability of cities. Ultimately, we expect to achieve a synergy between public libraries and cities in terms of sustainability. We found that the most dominant element in the exterior space of Japanese public libraries is parking lots, which have the potential to be flexibly utilized in the post-pandemic era. We also summarized the current patterns of external space for public libraries in Japan. Finally, we proposed six strategies to enhance the sustainability of libraries and cities. These strategies can simultaneously enhance the sustainability of cities and public libraries from multiple perspectives, especially in the post-pandemic era. Our proposal will not only help to build or renew public libraries in the future, but also fills a gap in the urban perspective of Japanese public libraries and their external spaces.