Cities worldwide are calling for smart mobility strategies to tackle the negative externalities of their transport networks. Mobility as a Service (MaaS) is expected to introduce a new mobility model that promotes smarter and more sustainable urban futures. Given the novelty of the concept, this paper explores the implications that might arise from the implementation of MaaS in today’s metropolises in relation to the six dimensions of smart cities: smart governance, smart economy, smart mobility, smart environment, smart people, and smart living. To this end, 42 semi-structured interviews with MaaS stakeholders were conducted in Madrid (Spain). Thematic analysis identified a set of 35 urban implications. The success of MaaS requires more than the mere deployment of technologies and must be supported by the reorganisation of institutional structures, the reform of the regulatory scenario, the stimulation of innovation-based entrepreneurship, the promotion of environmental awareness, the encouragement of cultural transition, and the consideration of the public sphere. Overall, valuable insights are identified for policymakers when designing MaaS. Assessment of multiple stakeholders’ perspectives enables a holistic understanding of these strategies and thus maximises their potential to address the limitations of our complex mobility systems in meeting urban sustainability goals.