MnGe thin films have been demonstrated as promising spin-injector materials for germanium-based spintronic devices. So far, MnGe has been grown epitaxially only on Ge (111) substrates. In this letter, we present the growth of epitaxial MnGe films on Ge (100) substrates. The MnGe film is synthetized via sub-second solid-state reaction between Mn and Ge upon flash lamp annealing for 20 ms at the ambient pressure. The single crystalline MnGe is ferromagnetic with a Curie temperature of 283 K. Both the c-axis of hexagonal MnGe and the magnetic easy axis are parallel to the Ge (100) surface. The millisecond-range flash epitaxy provides a new avenue for the fabrication of Ge-based spin-injectors fully compatible with CMOS technology.