Updated project metadata. During metaphase, in response to improper kinetochore-microtubule attachments, the spindle assembly checkpoint (SAC) activates the mitotic checkpoint complex (MCC) to inhibit the anaphase-promoting complex/cyclosome (APC/C). The Mad1-Mad2 complex provides a catalytic platform for MCC assembly. Mad1-bound Mad2 recruits open-Mad2 through asymmetric dimerization and Mad1 phosphorylation by Mps1 promotes conversion of Mad2 from an open (O-Mad2) to closed (C-Mad2) state, which binds Cdc20 to form the MCC. How Mad1 phosphorylation catalytically activates MCC formation is poorly understood. This study characterises Mad1 phosphorylation by Mps1 and provides structural and biochemical insights into a phosphorylation-specific Mad1-Cdc20 interaction, which allows for a tripartite assembly of Bub1-Mad1-Cdc20 on the C-terminal domain of Mad1. We also identify a folded state of Mad1-Mad2 complex, suggesting a model by which the Cdc20-Mad1 interaction brings the Cdc20 MIM motif near Mad2. The Cdc20 MIM motif is then entrapped by the Mad2 safety belt to form a stable complex, allowing spontaneous MCC assembly.