A novel mid-infrared (MIR) laser crystal Co/Er:PbF2 was successfully grown. The use of Er3+ ion co-doping to sensitize a Co2+ ion and enhance the 2.1-4.2 µm broadband MIR emission of the Co2+ ion in a PbF2 crystal was studied for the first time, to the best of our knowledge. The Er3+ ion was demonstrated to be an effective sensitizer of the Co2+ ion, making the Co/Er:PbF2 crystal propitious to be pumped by commercialized laser diodes. Furthermore, with Er3+ ion co-doping, the local symmetry of Co2+ and Er3+ ions was seriously distorted, thereby enhancing the 2.1-4.2 µm MIR emissions. This study provides a path for designing MIR laser materials with optimal performance.