Room-temperature X-ray diffraction, piezoresponse force microscopy, and SQUID magnetometry measurements of the Bi0.9Ca0.1Fe1-y Mn (y) O-3 (0 a parts per thousand currency sign y a parts per thousand currency sign 0.5) ferromanganites have been carried out to illustrate the effect of B-site substitution on the crystal structure and multiferroic properties of the Ca-doped compound representing an intermediate ferroelectric and weak ferromagnetic phase of the Bi1-x Ca (x) FeO3-x/2 perovskites. The Mn doping has been shown to restore multiferroic behavior specific to pure BiFeO3. Indeed, the 0.1 a parts per thousand currency sign y a parts per thousand currency sign 0.4 samples have been found to possess a single-phase rhombohedral structure compatible with the ferroelectric polarization and antiferromagnetism. Further increase of the Mn concentration stabilizes an orthorhombic structure typical of the high-pressure antiferroelectric phase of the BiFe1-y Mn (y) O-3 perovskites. These results, particularly important from the viewpoint of understanding the origin of weak ferromagnetism in the Bi1-x Ca (x) FeO3-x/2 system, are discussed using a model accounting for the doping-related defect formation.