Ex vivo activation is a prerequisite to reaching adequate levels of gene editing by homology-directed repair (HDR) for hematopoietic stem and progenitor cell (HSPC)-based clinical applications. Here, we show that shortening culture time mitigates the p53-mediated DNA damage response to CRISPR-Cas9-induced DNA double-strand breaks, enhancing the reconstitution capacity of edited HSPCs. However, this results in lower HDR efficiency, rendering ex vivo culture necessary yet detrimental. Mechanistically, ex vivo activation triggers a multi-step process initiated by p38 mitogen-activated protein kinase (MAPK) phosphorylation, which generates mitogenic reactive oxygen species (ROS), promoting fast cell-cycle progression and subsequent proliferation-induced DNA damage. Thus, p38 inhibition before gene editing delays G1/S transition and expands transcriptionally defined HSCs, ultimately endowing edited cells with superior multi-lineage differentiation, persistence throughout serial transplantation, enhanced polyclonal repertoire, and better-preserved genome integrity. Our data identify proliferative stress as a driver of HSPC dysfunction with fundamental implications for designing more effective and safer gene correction strategies for clinical applications.