The cell-cycle regulated methyltransferase SET8 is the sole enzyme responsible for the mono-methylation of histone H4 at lysine 20 (H4K20) that is the substrate for di- and trimethylation mainly by SUV4-20Hs enzymes. Both SET8 and SUV4-20Hs have been implicated in regulating DNA repair pathway choice through the inverse affinities of BRCA1-BARD1 and 53BP1 complexes for disparate methylation states of H4K20. However, the precise and respective functions of each H4K20 methyltransferase in DNA repair pathways remain to be clarified. Here, we show that SET8 acts as a potent chromatin inhibitor of homologous recombination and that its timely degradation during DNA replication is essential for the spontaneous nuclear focal accumulation of BRCA1 and RAD51 complexes during the S phase. Strikingly, the anti-recombinogenic function of SET8 is independent of SUV4-20H activity but requires the subsequent recruitment of the ubiquitin ligase RNF168. Moreover, we show that SET8-induced BRCA1 inhibition is not necessarily related to the loss of BARD1 binding to unmethylated histone H4K20. Instead, it is largely caused by the accumulation of 53BP1 in a manner depending on the concerted activities of SET8 and RNF168 on chromatin. Conversely, the lack of SET8 and H4K20 mono-methylation on newly assembly chromatin after DNA replication led to the untimely accumulation of BRCA1 on chromatin at the subsequent G1 phase. Altogether, these results establish the de novo activity of SET8 on chromatin as a primordial epigenetic lock of the BRCA1-mediated HR pathway during the cell cycle.