Voltage regulation in medium voltage (MV) and low voltage (LV) distribution networks has always been considered and approached as separate issues. However, changes in voltage magnitudes in an MV network can affect voltage magnitudes in an associated LV network and vice versa. In addition, voltage changes in an MV network can be contrary to changes in the associated LV network. This paper presents a voltage regulation methodology for MV and LV networks as a combined, network-wide problem. A theoretical formulation is developed to present MV and LV networks in a holistic manner, applicable for the development of an efficient voltage regulation approach. The proposed formulation is tested on a modified IEEE 13 node test feeder and verified using OpenDSS. The results confirm that the proposed formulation can successfully solve both MV and LV networks simultaneously. Based on the developed theoretical formulation, this paper proposes an integrated volt/var control philosophy to perform voltage regulation in both MV and LV networks jointly. The proposed volt/var control method is tested on the same test feeder over a 24-h period with varying load and generation. The results confirm that the proposed volt/var control method can successfully improve voltage profiles across both MV and LV networks in comparison to using a conventional volt/var control method.