<p>Cation identity and concentration strongly influence electrocatalytic processes, yet their effects remain insufficiently understood. Taking hydrogen evolution reaction in alkaline media as a model system, variations in cation concentration induce complex, sometimes inverted, activity trends. Increasing cation concentration can either promote or inhibit electrocatalytic activity depending on cation identity, electrode material and solution pH. These Janus-faced effects of cations challenge the current understandings of cation effects in electrocatalysis, which typically emphasize either promotional or inhibitory roles. Here, we propose a mechanistic rationale for the promoter-inhibitor transitions of cation effects and identify cation position in the electric double layer as the key factor governing this behavior. The theoretical framework distinguishes two cation states: cations electrostatically attracted in the diffuse layer, or cations specifically adsorbed at the inner Helmholtz plane. Incorporating the electric field effect on water dissociation beyond the Frumkin corrections, we show that the two cation states modulate the local electric field and thus kinetics in opposite ways. The observed inversions result from their competition, governed by cation size and adsorption strength. The framework and insights will be relevant to other electrocatalytic reactions at strongly negatively charged surfaces, such as CO₂ reduction.</p>