In response to DNA damage, checkpoint proteins halt cell cycle progression and promote repair or apoptosis, thereby preventing mutation accumulation and suppressing tumor development. The DNA damage checkpoint protein Hus1 associates with Rad9 and Rad1 to form the 9-1-1 complex, which localizes to DNA lesions and promotes DNA damage signaling and repair. Because complete inactivation of mouse Hus1 results in embryonic lethality, we developed a system for regulated Hus1 inactivation in the mammary gland to examine roles for Hus1 in tissue homeostasis and tumor suppression. Hus1 inactivation in the mammary epithelium resulted in genome damage that induced apoptosis and led to depletion of Hus1-null cells from the mammary gland. Conditional Hus1 knockout females retained grossly normal mammary gland morphology, suggesting compensation by cells that failed to undergo Cre-mediated Hus1 deletion. p53-deficiency delayed the clearance of Hus1-null cells from conditional Hus1 knockout mice and caused the accumulation of damaged, dying cells in the mammary gland. Notably, compensatory responses were impaired following combined Hus1 and p53 loss, resulting in aberrant mammary gland morphology and lactation defects. Overall, these results establish a requirement for Hus1 in the survival and proliferation of mammary epithelium and identify a role for p53 in mammary gland tissue regeneration and homeostasis.