The electronic structure of Mg and free-base tetraphenylporphyrin films on Ag(100) is investigated by one-and two-photon photoemission in combination with electronic structure calculations using density functional theory and the self-consistent GW(0) method. We determine the two highest occupied and the nearly degenerate lowest unoccupied molecular orbitals. Higher unoccupied states are seen in an enhanced emission as a final-state effect. For photon energies close to the prominent absorption of the Soret band we observe a strong electron emission attributed to the break up of the bound electron-hole pairs in the S-2 excited state. The experimental results on the occupied and unoccupied energy levels for the molecular films on Ag(100) nicely agree with calculated quasiparticle energies and experiments of the molecules in the gas phase.