ABSTRACT Post-starburst (PSB) galaxies belong to a short-lived transition population between star-forming (SF) and quiescent galaxies. Deciphering their heavily discussed evolutionary pathways is paramount to understanding galaxy evolution. We aim to determine the dominant mechanisms governing PSB evolution in both the field and in galaxy clusters. Using the cosmological hydrodynamical simulation suite Magneticum Pathfinder, we identify 647 PSBs with z ∼ 0 stellar mass $M_* \ge 5 \times 10^{10} \, \mathrm{M_{\odot }}$ . We track their galactic evolution, merger history, and black hole activity over a time-span of $3.6\,$ Gyr. Additionally, we study cluster PSBs identified at different redshifts and cluster masses. Independent of environment and redshift, we find that PSBs, like SF galaxies, have frequent mergers. At z = 0, $89{{\ \rm per\ cent}}$ of PSBs have experienced mergers and $65{{\ \rm per\ cent}}$ had at least one major merger within the last $2.5\,$ Gyr, leading to strong star formation episodes. In fact, $23{{\ \rm per\ cent}}$ of z = 0 PSBs were rejuvenated during their starburst. Following the mergers, field PSBs are generally shutdown via a strong increase in active galactic nucleus (AGN) feedback (power output $P_{\rm AGN,PSB} \ge 10^{56}\,$ erg Myr−1). We find agreement with observations for both stellar mass functions and z = 0.9 line-of-sight phase space distributions of PSBs in galaxy clusters. Finally, we find that z ≲ 0.5 cluster PSBs are predominantly infalling, especially in high-mass clusters and show no signs of enhanced AGN activity. Thus, we conclude that the majority of cluster PSBs are shutdown via an environmental quenching mechanism such as ram-pressure stripping, while field PSBs are mainly quenched by AGN feedback.