Linker histone H1.2 has been shown to suppress p53-dependent transcription through the modulation of chromatin remodeling; however, little is known about the mechanisms governing the antagonistic effects of H1.2 in DNA damage response. Here, we show that the repressive action of H1.2 on p53 function is negatively regulated via acetylation of p53 C-terminal regulatory domain and phosphorylation of H1.2 C-terminal tail. p53 acetylation by p300 impairs the interaction of p53 with H1.2 and triggers a rapid activation of p53-dependent transcription. Similarly, DNA-PK-mediated phosphorylation of H1.2 at T146 enhances p53 transcriptional activity by impeding H1.2 binding to p53 and thereby attenuating its suppressive effects on p53 transactivation. Consistent with these findings, point mutations mimicking modification states of H1.2 and p53 lead to a significant increase in p53-induced apoptosis. These data suggest that p53 acetylation-H1.2 phosphorylation cascade serves as a unique mechanism for triggering p53-dependent DNA damage response pathways.