Organic polymers are generally regarded as thermal insulators because of their random arrangement of polymeric chains that lead to scattering of heat-conducting phonons. We previously found that highly oriented assemblies composed of M13 phage, a filamentous virus, showed high thermal diffusivity, even though phonons are conducted on noncovalent bonds. However, biomolecular M13 phages did not allow utilization of organic solvents, resulting in limited applicability. Here, we utilized chemically modified M13 phage with oligo(ethylene glycol) (OEG) to improve its solubility and resistance to organic solvents, and to expand the applicability of phage-based thermally conductive assemblies. The high thermal diffusivity of assemblies composed of M13 phages modified with longer EG chains (EG unit: 10) was maintained when the assemblies were prepared using a mixed solvent of water and tetrahydrofuran, whereas that of assemblies composed of unmodified and shorter EG chain (3 and 6)-modified M13 phages was not maintained. When the mixed solvent was used, structural characterization revealed the presence of ordered and hexagonally packed structures that formed ordered assemblies, leading to phonon-conductive assemblies in the case of the longer EG chain-modified M13 phages. Our results will contribute to the construction of novel thermally conductive soft materials composed of biomacromolecular assemblies using organic solvents on substrates with complex surface morphologies and/or hydrophobic surfaces.