Ataxia telangiectasia and Rad3-related (ATR) and checkpoint kinase 1 (Chk1) are crucial kinases in the DNA damage response (DDR) pathway. While the roles of ATR and Chk1 within the DDR are well established, their roles in mitosis are not fully understood. Here, we describe that the ATR-Chk1 pathway is rewired during mitosis to promote full CDK1 activity, starkly contrasting its role in interphase, where it inhibits CDK1 following DNA damage in human cells. In mitosis, Chk1 inhibits residual activity of PKMYT1 (Myt1) via direct phosphorylation at Serine 143. Partial loss of CDK1 activity caused by inhibition of mitotic Chk1 leads to different effects on mitotic progression than full CDK1 inhibition. It causes increased lagging chromosomes in part through loss of Aurora B activity. Thus, mitosis-specific ATR-Chk1 activity is necessary to promote faithful chromosome segregation by ensuring that CDK1 activity is maintained in mitosis.