More than one hundred and fifty cities around the world have expanded their emergency cycling and walking infrastructures to increase their resilience in the face of the COVID-19 pandemic. In addition, the role of mobile apps is prominent in respect to developing a smart city during this pandemic, which raises the questions of how mobile apps contribute to the improvement of walking/cycling behavior and how such a relationship is influenced by the situation imposed by COVID-19. The role of mobile apps in the three relevant fields of physical activity, transport, and urban planning are reviewed. Next, the associations between walking/cycling behaviors and their contributing factors and the impacts of the COVID-19 pandemic on these relationships are reviewed. Studies on physical activity have emphasized the role of motivational social factors in improving the function of mobile apps. In regard to transport, mobile apps have the potential to facilitate data collection in macroscale environments. In addition, mobile apps may facilitate people’s recognition of positive/negative environmental aspects, and this may in turn lead to greater pedestrian/cyclists’ awareness and better organization of their walking/cycling behavior. Moreover, based on a participatory approach, the classification of current mobile apps and certain suggestions on the development of future mobile apps are presented. Finally, complementary suggestions are provided for maintaining and improving the use of mobile apps to improve the level of walking/cycling.