Waveguiding structures consisting of dielectric-loaded rectangular waveguides are important both in their own right and as a model for other configurations of much interest in the millimetre-wave area. Discontinuities and obstacles in such guiding structures provide the circuit elements used in microwave device design. Accurate prediction of device behaviour requires evaluation of the fields in the system. An increasingly used mathematical formulation is that of the integral equations approach, for which the Green's function of the structure is required. The development of the complete electric dyadic Green's function for an inhomogeneously loaded waveguide, which has not yet appeared in the literature is presented. The components of the dyadic are found in the form of eigenfunction expansions.