This study aims to understand the mechanisms of the activation and evolution of the marine heatwave (MHW) that occurred in the Gulf of Mexico (GOM) during summer 2023. We quantified contributions of the thermodynamic processes that transformed surface waters in the GOM into an unprecedented large volume of extremely warm water (> 31.8^oC). Through water mass transformation analysis of reanalyses data, we find that the genesis of this MHW was due to the compounding effect of anomalously warm winter surface water priming the region for a MHW, coupled with greater exposure to strong solar radiation. Transformation due to total surface fluxes (sensible and latent heat, solar and longwave radiation) contributed to the MHW volume at a peak rate of 17.7 Sv (Sv = 10⁶ m³ s^-1), while mixing countered the effect by 14.6 Sv at its peak. Total transformation during this 2023 MHW peaked at 4.9 Sv.