The advent of nanotechnology in the 21st century opened a new branch of nanoscience known as nanofluids, finding a wide range of industrial applications especially in heat transfer. Though the theory of thermal conductivity of solids is well established, there is no such conclusive model to explain the thermal conductivity of nanofluids. In the present work we propose a generalized theory for thermal conductivity applicable to materials ranging from heterogeneous solids, porous materials, nanofluids, and ferrofluids. The model could explain the effective thermal conductivity of not only the combination of solids but also solid–fluid mixtures. The proposed theory could successfully link the existing models for porous solid materials and nanofluids as its special cases. The proposed model is verified against experimental data by simulating the theoretical equations.