The increasing scarcity of potable water, exacerbated by climate change, population growth, and urbanization, calls for sustainable alternatives in sectors with high water consumption, such as concrete production. This study explores the feasibility of replacing drinking water with alternative sources, including rainwater, river water, treated wastewater, and concrete slurry water, in the preparation of cement mortars. Five water types were tested for their chemical and physical characteristics and their influence on the fresh, mechanical, and durability performance of mortars. Despite differences in pH, conductivity, and ionic composition, all water types complied with EN 1008 standards. Mortars were assessed for workability, compressive and flexural strength, chloride permeability, carbonation, freeze–thaw resistance, and steel rebar corrosion. The results showed that alternative waters can be used without significantly compromising mortar performance. Slight differences were noted in early strength and chloride permeability, especially in mortars mixed with treated wastewater, but long-term compressive strength remained consistent across all mixes. The study highlights the resilience of cementitious systems to variations in water chemistry and supports the sustainable use of non-potable waters in construction, particularly in regions facing water stress.