Adeno-associated virus (AAV) DNA, by mimicking a stalled replication fork, provokes a DNA damage response that can arrest cells in the G2/M phase of the cell-cycle. This response depends strictly on DNA damage signaling kinases ATR and Chk1. Here, we used AAV to study long-term effects of DNA damage signaling in cells with altered p53 status. In HCT116 cells, in response to damage signaling, p53 represses transcription of the genes encoding mitotic regulators Cdc25C, cyclin B1, and Plk1 to establish a firm G2 arrest. Isogenic cells lacking p53 maintain these three proteins at constant levels yet can still arrest initially in G2 because Chk1 signaling inhibits their enzymatic activities. Unexpectedly, the levels of Chk1 fall abruptly in a proteasome-dependent manner after two days of arrest in G2. In p53-deficient cells, this Chk1 instability is coupled to recovery of the phosphatase activity of Cdc25C and in the kinase activities of Plk1 and Cdk1/cyclin B1. Consequently, the p53-deficient cells enter lethal mitosis. Thus, the Chk1-mediated arrest is transient: it initially causes cells to accumulate in G2 until p53-dependent transcriptional repression of mitotic proteins takes over. p53-deficient cells cannot maintain the DNA damage signaling-induced G2 arrest after Chk1 has disappeared, and continue into catastrophic mitosis. Restoring Chk1 prevents the cells from entering such mitosis. These results reveal a mechanism based on Chk1 stability that regulates mitotic entry after DNA damage and elucidate the controversial phenomenon of p53-promoted cell survival in the face of damage signaling.