This study reports the synthesis and comprehensive characterization of layered solid solutions in the GeBi2Te4–MnBi2Te4 and GeBi4Te7–MnBi4Te7 systems, which belong to the tetradymite family of compounds known for their thermoelectric and topological insulator properties. Alloys with varying Ge−Mn ratios were prepared using a two-stage thermal treatment involving melt quenching followed by prolonged annealing. Powder X-ray diffraction confirmed the formation of single-phase solid solutions across the full compositional range in both systems. A systematic shift in diffraction peaks with increasing Mn content indicates lattice parameter variation consistent with Vegard’s law, reflecting the substitution of Ge2+ by larger Mn2+ ions. Le Bail refinement was employed to determine precise lattice parameters, revealing smooth trends without structural anomalies. Scanning electron microscopy further confirmed the microstructural homogeneity of the samples. The findings demonstrate the formation of continuous solid solutions in both systems, offering a basis for future exploration of their electronic and magnetic properties.