A local-density approximation with the Hubbard U correction (LDA+U) method has been employed to investigate oxygen vacancy formation and migration in CexTh1-xO2. The addition of CeO2 into ThO2 significantly decreases the oxygen vacancy formation and migration energies. ThO2 containing 50% CeO2 exhibits the lowest calculated formation energy, 3.7 eV, and the lowest calculated migration energy, 0.2 eV, occurs for a CeO2 content of 75%, suggesting that introducing CeO2 into ThO2 promotes the formation of mobile oxygen vacancies. If the ceria content is less than about 35%, the reduced CexTh1-xO2 becomes antiferromagnetic (AFM), whereas the ferromagnetic (FM) state dominates for x values above about 35%, which may allow the tailoring of magnetic properties by varying the CeO2 content.