Abstract Bacillus subtilis ParB forms multimeric networks involving non-specific DNA binding leading to DNA condensation. In our previous work (Fisher et al., 2017), we found that an excess of the free C-terminal domain (CTD) of ParB impeded DNA condensation or promoted decondensation of pre-assembled networks. However, interpretation of the molecular basis for this phenomenon was complicated by our inability to uncouple protein binding from DNA condensation. Here, we have combined lateral magnetic tweezers with TIRF microscopy to simultaneously control the restrictive force against condensation and to visualize ParB protein binding by fluorescence. At non-permissive forces for condensation, ParB binds non-specifically and highly dynamically to DNA. Our new approach concluded that the free CTD blocks the formation of ParB networks by heterodimerization with full length DNA-bound ParB. This strongly supports a model in which the CTD acts as a key bridging interface between distal DNA binding loci within ParB networks. Significance Statement Using combined Magnetic Tweezers and TIRF microscopy we show that the CTD of ParB blocks ParB network formation by heterodimerization with the full-length protein, which remains bound to the DNA.