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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°C)$( > 31.8{}^{\circ}\mathrm{C})$. 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 17.0 Sv (106${10}^{6}$m3${\mathrm{m}}^{3}$s−1${\mathrm{s}}^{-1}$= Sv), while the residual term (including mixing) countered the effect by 22.3 Sv at its peak. Total transformation during this 2023 MHW peaked at 4.9 Sv. The goal of this study is to understand what triggered the marine heatwave (MHW) that occurred in the Gulf of Mexico during summer 2023. We investigate this MHW by analyzing what physical processes transform normal water into water with extremely high temperatures and vice versa. This is done by tracking the volume, and its changes, of the very warm water. We find that the 2023 MHW was activated by the ocean surface waters being much warmer than normal in the winter of 2022. This was intensified by increased exposure to summer solar radiation. Other processes, such as mixing, acted to counter the warming effects; however, the counter processes were ultimately no match for the strong heating at the surface, yielding to the activation and persistence of an extreme heat event. In July 2023, an unprecedentedly large volume of extremely warm water formed during a marine heatwave in the Gulf of MexicoWater Mass Transformation provides a novel quantification of the physical processes that contributed to the creation of this marine heatwaveThe 2023 marine heatwave was caused by the anomalously high absorption of solar radiation within anomalously warm surface waters In July 2023, an unprecedentedly large volume of extremely warm water formed during a marine heatwave in the Gulf of Mexico Water Mass Transformation provides a novel quantification of the physical processes that contributed to the creation of this marine heatwave The 2023 marine heatwave was caused by the anomalously high absorption of solar radiation within anomalously warm surface waters