Spectral sensitization of a mesoporous graphite carbon nitride (mpg-C3N4) photocatalyst was investigated by depositing magnesium phthalocyanine (MgPc) to expand the absorption to wavelengths longer than those of the principal mpg-C3N4. The obtained sample, MgPc/Pt/mpg-C3N4 (Pt as a cocatalyst) showed stable photocatalytic evolution of hydrogen from aqueous solution in the presence of sacrificial reagents (triethanolamine), even under irradiation at wavelengths longer than 600 nm. Increasing the amount of MgPc led to ordered MgPc aggregation on the photocatalyst surfaces. The rate of photocatalytic hydrogen evolution was highest on a sample with an amount of MgPc corresponding to a monolayer on the Pt/mpg-C3N4 photocatalyst surface. The obtained action spectra of hydrogen evolution and the observation that the amount of evolved hydrogen substantially surpassed the amount of MgPc, confirm that the introduced MgPc functioned as a photocatalytic sensitizer.