We present the experimental demonstration and interpretation of an ultrafast optically tunable, graphene-based thin film absorption modulator for operation in the THz regime. The graphene-based component consists of a uniform CVD-grown graphene sheet stacked on a SU-8 dielectric substrate which is grounded by a metallic ground plate. The structure shows enhanced absorption originated from constructive interference of the impinging and reflected waves at the absorbing graphene sheet. The modulation of this absorption, which is demonstrated via a THz time-domain spectroscopy setup, is achieved by applying an optical pump signal which modifies the conductivity of the graphene sheet. We report an ultrafast (of the order of 2.79~ps) absorption modulation in the order of 40\% upon photoexcitation. Our results provide evidence that the optical pump excitation results in the degradation of the graphene THz conductivity, which is connected with the generation of hot carriers, the increase of the electronic temperature and the dominant increase of the scattering rate over the carrier concentration as found in highly doped samples.