Because deformed magnesium alloys with low density and desirable mechanical properties can save energy and reduce emissions, they are in considerable demand in industrial applications. However, due to their hcp crystal structure, magnesium alloys have a limited number of slip systems that can be activated at room temperature. At present, many deformed magnesium alloys still face the problems of insufficient formability and poor mechanical anisotropy at room temperature, which limit the development of secondary forming processes. Meanwhile, many reports have proven that regulating the texture of deformed magnesium alloys is an effective strategy to improve their formability. Therefore, this study primarily employs the neutron diffraction technology, combined with microscopic characterization methods such as SEM and EBSD, to systematically study the effects of the minor rare earth element Ce on the microstructure, bulk texture, and mechanical anisotropy of extruded Mg-0.3Al-0.2Ca-0.5Mn magnesium alloy sheet. The results show that as the Ce content increases, second-phase particles in the extruded magnesium alloy sheet gradually transform from Al₈Mn₅ to Ce-containing Al₈Mn₄Ce and Al₁₁Ce₃ particles and the number density of second-phase particles increases remarkably. The addition of 0.05%Ce (mass fraction) did not considerably improve the basal texture of the sheet. However, after the addition of 0.5%Ce, fiber texture components distributed along the transverse direction (TD) are substantially reduced while the texture of (0002) pole figure of the alloy changed from a bimodal basal texture to a bimodal nonbasal texture along the extrusion direction (ED). This nonbasal texture formation is mainly caused by larger Ce-containing second-phase particles (> 1 μm) promoting the particle-stimulated nucleation effect and preferential growth of nonbasal-oriented recrystallized grains. Therefore, when 0.5%Ce is added, the basal texture of the alloy is remarkably optimized, so that the ratio of the tensile yield strength along TD to that along ED is close to 1, implying that the anisotropy of the extruded Mg-0.3Al-0.2Ca-0.5Mn alloy has been substantially reduced by the minor addition of Ce element.