Electrochemical CO2 reduction at high current densities is often limited by electrolyte flooding and competitive hydrogen evolution reaction (HER) on catalysts with insufficient surface hydrophobicity. Herein, we report organic-ligand-free in situ formed mesoporous Ag nanowires (Ag NWs), in which the porous architecture maintains a morphology-driven partial-wetting state that provides abundant electrolyte-accessible active sites while preserving efficient CO2 transport pathways. To achieve this structure, we develop a facile nanostructuring strategy based on sequential vapor-phase iodination and controlled electrochemical reduction of Ag. We reveal that the reduction rate governs the growth pathway and determines the architecture of catalysts: slow reduction promotes anisotropic nanowire formation, whereas rapid reduction yields short, randomly oriented Ag nanodendrites. The in situ formed Ag NWs achieve a high CO Faradaic efficiency of 98.9% at −1.06 V vs. the reversible hydrogen electrode (RHE) and maintain stable performance for over 300 h at a current density of 100 mA cm−2, while sustaining a low H2 Faradaic efficiency of 2.63% even at 400 mA cm−2. These findings demonstrate the critical role of mesoporous surface architectures in regulating interfacial wetting and CO2 mass transport, establishing a facile and broadly applicable strategy for designing high-performance Ag-based CO2 reduction electrocatalysts.