Re-induction of endogenous proliferation is a promising regeneration strategy for post-mitotic organs. For heart regeneration, we recently discovered that dual inhibition of both GSK3 and MST1 is required for proliferation in mature human cardiac organoids1. However, the mechanisms of action are not yet understood. Here, we de-convolute the mitogenic response using proteomics and report that GSK3 inhibition activates a cell cycle network whereas MST1 inhibition drives the mevalonate pathway. Screening of an additional 105 compounds identified a p38 inhibitor, which activated a cell cycle network, as well as an inhibitor of TGFBR, which activated the mevalonate pathway; combinatorial treatment with these two inhibitors also resulted in synergistic cell cycle activation. The mevalonate pathway which was consistently activated under the most robust proliferative conditions, is down-regulated during in vivo maturation when cardiomyocyte regenerative capacity ceases and inhibition of the mevalonate pathway abolished the myocyte proliferative response to mitogens. Taken together, our findings suggest that the mevalonate pathway synergises with mitogenic agents to enable mature cardiomyocytes to re-enter the cell cycle. These findings have important ramifications for development of cardiac regenerative therapeutics.