In the pursuit of developping efficient and green methods for hydrogen production, a key focus is the development of the most energy-efficient catalysts. The analysis of various proton sources primarily aims at eliminating rate-limiting steps associated with protonation events and ensuring the stability of the catalyst. In this work, we report how two distinct proton sources can cause a mechanistic shift in the hydrogen evolution reaction. Specifically, we explore this reactivity change in the presence of triethylammonium and trifluoroacetic acid with two thiosemicarbazone-based complexes, using cobalt and nickel metal centers. Our combined experimental and theoretical results reveal that the complete sequence of steps leading to hydrogen release strongly depends on the proton source. This demonstrates the importance of thoroughly investigating the interactions between a catalyst and a proton source to optimize hydrogen evolution systems.