The current paradigm states that exit from mitosis is triggered by the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) acting in concert with an activator called CDC20. While this has been well established for a number of systems, the evidence of a critical role of CDC20 in somatic cells is not unequivocal. In this study, we reexamined whether mitotic exit can occur properly after CDC20 is depleted. Using single-cell analysis, we found that CDC20 depletion with small interfering RNAs (siRNAs) significantly impaired the degradation of APC/C substrates and delayed mitotic exit in various cancer cell lines. The recruitment of cyclin B1 to the core APC/C was defective after CDC20 downregulation. Nevertheless, CDC20-depleted cells were still able to complete mitosis, albeit requiring twice the normal time. Intriguingly, a high level of cyclin-dependent kinase 1 (CDK1)-inhibitory phosphorylation was induced during mitotic exit in CDC20-depleted cells. The expression of an siRNA-resistant CDC20 rescued both the mitotic exit delay and the CDK1-inhibitory phosphorylation. Moreover, the expression of a nonphosphorylatable CDK1 mutant or the downregulation of WEE1 and MYT1 abolished mitotic exit in CDC20-depleted cells. These findings indicate that, in the absence of sufficient APC/C activity, an alternative mechanism that utilized the classic inhibitory phosphorylation of CDK1 could mediate mitotic exit.