Tetra(-4-pyridyl)porphyrinatomanganese(III)sulfate (as an MnIII+MnII ion mixture) was embedded into a polysiloxane polymer matrix and attached to the surfaces of the n-GaAs electrode. The n-GaAs/polymer/MnP system was annealed under nitrogen and used for a photoelectrochemical study in a water/LiClO4/Fe(CN)63−/Fe(CN)64− system. The values of short-circuit currents, measured after minutes of illumination, were significantly enhanced by modification. The modified electrode surfaces were more stable to degradation, in the dark and under illumination, than the unmodified ones. Furthermore, the modified electrodes showed higher light-to-electricity conversion efficiency than the unmodified ones. The methodology described here is advantageous in the sense that the semiconductor electrode properties can be enhanced in more than one aspect at the same time.