New solid solutions Cu2MnxFe1−xSnS4 were prepared by direct combination of the corresponding elements at 850 °C. The crystal structure of Cu2Mn0.4Fe0.6SnS4 was determined by single-crystal X-ray diffraction. This phase is described in the space group math formula where each cation is tetrahedrally coordinated to four sulfur anions in a sphalerite-like arrangement. The XRD patterns of the solid solutions Cu2MnxFe1−xSnS4 were fully indexed in the space group math formula and the values of the cell parameter for all phases obey the usual linear Vegard behavior. A progressive evolution of the magnetic moment in the paramagnetic state is observed when increasing the content of manganese. The negative values of the Curie–Weiss constant, θ, indicate an antiferromagnetic (AF) behavior with AF interactions, weaker by more than one order of magnitude compared to other diluted magnetic semiconductors (DMSs) with zinc-blende or wurzite crystal structure.