Expansive clays, such as bentonite, are widely used in geo-environmental systems due to their low permeability, swelling potential, and self-sealing capacity. Their behaviour under varying mechanical and environmental conditions remains difficult to predict because of the strong coupling between hydraulic and mechanical processes. Experimental evidence on structural changes in expansive clays subjected to compaction and wetting under constant volume shows that micro- and macropore domains evolve and alter the balance between adsorptive and capillary retention mechanisms. This structural evolution, as well as water retention hysteresis, directly affects the mechanical response, yet these factors are often overlooked in modelling approaches. Even double-structure formulations, despite involving a large number of parameters and requiring extensive calibration, often fail to capture all these aspects. To address these limitations, ACMEG-Ex-S, an extension of the ACMEG-S stress–strain framework specifically developed for expansive clays, is proposed. The formulation independently describes capillary and adsorptive retention, incorporates hysteresis, and captures structural evolution under hydraulic and mechanical loading. A void ratio-dependent contribution factor and a void ratio-dependent air entry value allow these effects to be embedded within a single-structure mechanical framework, reducing complexity and the number of required mechanical parameters. Validation against MX-80 bentonite experiments, including swelling pressure, oedometric and isotropic compression tests under various hydro-mechanical conditions, demonstrates that the formulation reproduces non-linear stress–strain responses, swelling behaviour, and hydro-mechanical coupling across diverse conditions.