Several compositions of manganese tin bismuth selenide solid solution series, Mn1-xSnxBi2Se4 (x = 0, 0.3, 0.75), were synthesized by combining high purity elements in the desired ratio at moderate temperatures. X-ray single crystal studies of a Mn-rich composition (x = 0) and a Mn-poor phase (x = 0.75) at 100 and 300 K revealed that the compounds crystallize isostructurally in the monoclinic space group C2/m (no.12) and adopt the MnSb2Se4 structure type. Direct current (DC) magnetic susceptibility measurements in the temperature range from 2 to 300 K indicated that the dominant magnetic ordering within the Mn1-xSnxBi2Se4 solid-solutions below SO K switches from antiferromagnetic (AFM) for MnBi2Se4 (x = 0), to ferromagnetic (FM) for Mn0.7Sn0.3Bi2Se4 (x = 0.3), and finally to paramagnetic (PM) for Mn0.25Sn0.75Bi2Se4 (x = 0.75). We show that this striking variation in the nature of magnetic ordering within the Mn1-xSnxBi2Se4 solid-solution series can be rationalized by taking into account: (1) changes in the distribution of magnetic centers within the structure arising from the Mn to Sn substitutions, (2) the contributions of spin-polarized free charge carriers resulting from the intermixing of Mn and Sn within the same crystallographic site, and (3) a possible long-range ordering of Mn and Sn atoms within individual {M}(n)Se4n+2 single chain leading to quasi isolated {MnSe6} octahedra spaced by nonmagnetic {SnSe6} octahedra.