Ce-doped LaMnO3 has complicated physical properties, such as unclear valence states of Ce and Mn ions, ferromagnetism–paramagnetism and metal-insulator transitions, and multiphase coexistence. To explore the ground-state properties of single-phase La1−xCexMnO3 and obtain better understanding of the complicated physical properties of the multiphase system, the electronic and magnetic structures of La1−xCexMnO3 are investigated by using the first principle calculations. The results show that single phase La1−xCexMnO3 is neither a hole-doped nor an electron-doped compound. The valence states of Ce and Mn ions are predicted to be trivalent in an ionic picture. Trivalent Mn ions energetically favor an A-type antiferromagnetic ordering and insulating band structure. In comparison with the experimental studies, it is concluded that overoxygenation and multiphase coexistence in experiments switch the ground state from an antiferromagnetic insulator to a ferromagnetic half-metal.