Shared Mobility Services (SMSs) have reshaped urban transportation systems, providing effective and flexible mobility options. As these services can reduce traffic congestion and emissions by reducing the number of cars, they are likely to become an essential part of the urban transportation landscape, which emphasizes the importance of analyzing and understanding their impacts on the system and users’ travel choices, especially when integrated with public transport (PT). Most existing works do not consider interactions between SMSs and PT, leading to inaccurate PT and SMS estimates, as in most cases, PT does not cover the first and last mile of journeys. In this study, we first provide a comprehensive literature review on modeling SMSs in multi-modal systems. Then, we present an analytical traffic assignment model formulated as a Mixed-Integer Quadratic Problem, which can be solved with existing solvers. In particular, the proposed model can represent various travel options and any logical combination. We adapt the Beckmann formulation with a link-wise objective function, including time and monetary costs. We perform an in-depth analysis of commuter behavior on two test cases and investigate the difference between user equilibrium and system optimum in such a complex system.