Mixed species biofilms have been able to generate current densities of over 20 A/m² in microbial fuel cells (MFCs), while also offering advantages in stability, substrate flexibility, and culture maintenance costs. To further increase the current density, a possible approach is to enhance the conductivity of biofilms and thereby the direct extracellular electron transfer within the biofilms and to the anode. While examination of conductivity in microbial samples is still in its relative infancy and conceptual models in terms of conductive mechanisms are still being developed and debated, our study has demonstrated that current-producing mixed species biofilms can exhibit high conductivity across non-conductive gaps. Differential responses observed over varying potentials also suggest that a redox driven conductivity mechanism is dominant in the tested mixed-species biofilms. Considering that some redox conductive organic polymer films possess negative magnetoresistance property (resistivity of a conductive material decreases as the magnetic field strength increases) and magnetic field may stimulate enzyme activity in certain anaerobic wastewater treatment systems, our mixed species biofilms were exposed to low intensity magnetic field to investigate the conductivity change and performance enhancement. Exposure of the biofilms to magnetic field has led to an over 100% increase in biofilm conductance and over 300% increase in current density. Application of the low intensity magnetic field also encouraged the enrichment of Geobacter spp. in biofilms. Short term application of magnetic field also increased the conductivity of anodic biofilms, resembling the negative magnetoresistivity that has not been observed in conductive biofilms before. These results not only enhance our understanding of the electrical conductivity in microbial aggregates, but also inform rational design and operation of microbial-based fuel cells.