Here we report on successful synthesis of Ge-doped single crystal diamond at high-pressure high-temperature (HPHT) conditions of 7.0 GPa and 1500-1900 °C from an Mg-Ge-C system. The systematic study of diamond crystallization processes reveals that addition of Ge to the Mg-C system suppresses the intensity of diamond spontaneous nucleation enabling synthesis of relatively large (2-3 mm) diamond crystals via seeded growth. Temperature is found to be a key factor controlling diamond crystallization mechanisms, morphological characteristics, and optical properties of the crystals. It is established that the diamond growth rate depends almost exponentially on the temperature increasing from 3.3 μm/h at 1500 °C to 4.2 mm/h at 1900 °C. Spectroscopic characterization of the synthesized diamonds reveals the abundance of the 2.06 eV Ge-V centers in the photoluminescence spectra confirming successful doping of the diamonds with germanium. The feasibility of the Mg-Ge-C system for growing bulk low-strain diamond doped with isotopically enriched germanium is demonstrated. The isotopic shift of the zero-phonon lines of the 2.06 eV center and the newly observed 1.747 eV center is found, proving the involvement of germanium atoms in the structure of the corresponding defects.