A significant enhancement in the hydrogen (H2) sensitivity as well as selectivity after aging for more than 40 days has been observed for a mixed-potential-type sensor using ZnO (+84 wt % Ta2O5) as the sensing electrode (SE) and yttria-stabilized zirconia (YSZ) as the solid electrolyte. The effect of the aging process in enhancing the sensing characteristics of the sensor using ZnO (+84 wt % Ta2O5)-SE was studied here by investigating the changes in the morphology, crystal structure, chemical surface state, and catalytic properties of the SE material before and after aging at 500 °C for 80 days. X-ray diffraction measurements confirmed that the crystal structure of the SE material was found to be unaffected by aging, while the morphological change observed via scanning electron microscopy imaging indicated a decrease in the porosity and an increase in the particle size after aging. A significant change, particularly in the binding energy of Ta 4f, was also observed for the SE material after long-term aging. Although the catalytic activities toward the anodic reaction of H2 and the other examined gases were moderately stable after aging, a significant decrease in the heterogeneous catalytic activity of the gas-phase reaction (oxidation) of H2 was observed. Such a trend presumably resulted in a higher fraction of H2 reaching the triple-phase boundary, where the electrochemical reactions generate a sensing signal (mixed potential), resulting in high H2 sensitivity as well as high H2 selectivity after long-term aging of the present sensor.