Entry into mitosis is driven by the phosphorylation of thousands of substrates, under the master control of Cdk1. During entry into mitosis, Cdk1, in collaboration with MASTL kinase, represses the activity of the major mitotic protein phosphatases, PP1 and PP2A, thereby ensuring mitotic substrates remain phosphorylated. For cells to complete and exit mitosis, these phosphorylation events must be removed, and hence, phosphatase activity must be reactivated. This reactivation of phosphatase activity presumably requires the inhibition of MASTL, however, it is not currently understood how or what deactivates MASTL. In this study, we identified that PP1 is associated with and capable of partially dephosphorylating and deactivating MASTL during mitotic exit. Using mathematical modelling we were able to confirm that deactivation of MASTL is essential for mitotic exit. Furthermore, small decreases in Cdk1 activity during metaphase are sufficient to initiate the reactivation of PP1, which in turn partially deactivates MASTL to release inhibition of PP2A and hence create a feedback loop. This feedback loop drives complete deactivation of MASTL, ensuring a robust switch-like activation of phosphatase activity during mitotic exit.