MRG15, a chromatin remodeling protein, plays a pivotal role in cellular senescence and proliferation. However, the precise roles and mechanisms of MRG15 in aging regulation remain unclear. Our research elucidates the distinct functions of MRG15's splice variants in aging. We find that MRG15L, contrary to the previously assumed MRG15S, accumulates with advancing age. Using histone peptide binding assays and protein interaction analysis, we demonstrate that MRG15L exhibits reduced affinity for histone H4 acetylation sites, thereby weakening CDK1 regulation, leading to G2/M phase arrest and promoting cellular senescence. During postnatal cardiac development, MRG15L expression increases and is linked to reduced regenerative capacity. Moreover, targeted knockout of MRG15L in mice enhances cardiac repair and regeneration following myocardial ischemia-reperfusion injury. These findings highlight MRG15L as a promising therapeutic target for age-related diseases, revealing its critical role in modulating aging pathways through alternative splicing.