Results are presented for the momentum dependent two-loop contributions of order $\mathcal{O}(\alpha_s\alpha_t}$ to the masses and mixing effects in the Higgs sector of the real MSSM. They are obtained in the Feynman-diagrammatic approach using a mixed on-shell/$\bar{DR}$ renormalization that can directly be matched onto the higher-order corrections included in the code FeynHiggs. Several two-loop diagrams entering the calculation are not known analytically. They are evaluated numerically with the program SecDec. The combination of the new momentum dependent two-loop contribution with the existing one- and two-loop corrections in the on-shell/$\bar{DR}$ scheme leads to an improved prediction of the light MSSM Higgs boson mass and a correspondingly reduced theoretical uncertainty.