The structural evaluation of rubble stone masonry structures (e.g. building walls and masonry dams) is a complex task due to the heterogeneity and uncertainty in the properties of the constituent materials, mortars and stones, and the known variability in stone geometry and positioning. Old masonry dams have an even higher complexity as the upstream and downstream faces have usually regular bricks for impervious purposes whereas their interior is usually made of rubble masonry. A micro-modeling strategy based on a detailed particle model (PM) that follows a discrete element method (DEM) has been applied in the modelling and evaluation of traditional stone masonry unreinforced walls, transversally confined walls and reinforced with sprayed micro-concrete layers with external steel mesh and transverse confinement walls. The studies have shown that DEM-based PMs performed well in reproducing the response of the ancient masonry under compression-loading and compression-shear loading conditions. In this work the proposed DEM-based PM has been extended to assess masonry dam stability analysis considering its complex inner structure and including both its mechanical and hydraulic behaviour. The masonry dam stability analysis is carried out by amplifying the hydrostatic loads, by gradually increasing the reservoir water level using 1m height increments, to simulate an overtopping scenario. Parametric studies are also carried out to assess a potential loss of cohesion due to the chemical and physical deterioration caused by water flow through the dam discontinuities. The results presented show that the adopted micro-modelling approach can be readily used in the assessment of old masonry dams stability safety, including possible loss of cohesion scenarios, and that the approach may be useful in the evaluation of retrofitting solutions.