Historic structures remain the most resilient and persistent monuments unlike modern constructions. In many countries, these were mainly built with materials that the modern-era construction industry has slowly abandoned due to the evolution of materials and new emerging technologies. Replacement of original heritage materials with modern materials has become a common mistake despite the indisputably proven resistance of these materials to deterioration factors. Unfortunately, contemporary historic masonry restoration frequently overlooks the critical aspect of material compatibility, leading to unsatisfactory outcomes and premature failure. Consequently, this has negative economic consequences. This paper presents results for the mortars of the Robben Island structures in Cape Town, South Africa (UNESCO World Heritage Site). The research assesses the mechanical properties of the historical masonries and mortars using non-destructive testing methods; rebound hammer (for testing masonry surface hardness) and ultrasound pulse velocity for repair mortars. Visual assessment revealed water accumulation on the plaster, leading to mould growth. The rebound hammer test revealed differences in surface hardness and quality. The Maximum Security Prison had the highest rebound values (up to 50 recorded on mainly stone within the masonry), indicating exceptionally hard stone, whereas the Pre-school showed moderately high values (ranging up to 45 with one wall being significantly stronger). The Medium Security Prison had uniformly moderate rebound values (generally 15–28). The original mortars’ rebound values ranged from 15–20, giving an estimated compressive strength of 12 MPa. However, the repair mortars crumbled during this test, with an average laboratory compressive strength of 0.47 MPa. These findings argue for sustainable and compatible materials that contribute to improved heritage conservation protocols in South Africa and abroad.