The effect of Mn doping on crystal structure and magnetic behavior of the polar antiferromagnetic and antipolar weak ferromagnetic phases of the Bi1−xLaxFeO3 system was investigated. Both reference compounds, Bi0.87La0.13FeO3 (space group R3c, tilting scheme (a−a−a−)] and Bi0.82La0.18FeO3 [space group Pnam, tilting scheme (a−a−c+)/(a−a−c−)], were shown to undergo the Mn doping induced structural phase transformation towards the average Imma (a−b0a−) symmetry. Magnetization measurements performed for the compounds with the close to maximal manganese concentration (y = 0.4) found complex temperature-dependent behavior that can be indicative of a spin-reorientation transition. Enhanced spontaneous magnetization characteristic of the Mn-enriched x = 0.13 and x = 0.18 samples at low temperature is consistent with a competitive character of the superexchange interactions peculiar to the Fe/Mn containing perovskite systems.