The increasing demand for sustainable construction materials has led to the exploration of natural fibers as reinforcement in hydraulic lime mortars (HLMs) for masonry restoration and strengthening. This study investigates the mechanical performance of HLMs reinforced with two different natural fibers–hemp and Spanish broom–focusing on the effect of fiber length. Experimental tests including three-point bending and uniaxial compression were carried out on mortars incorporating fibers of varying lengths. The results demonstrate that optimal fiber length significantly enhances mechanical properties, particularly tensile strength and ductility. Hemp fibers performed best at lengths of 1.0–1.5 cm, while Spanish broom fibers exhibited superior post-crack toughness at 2.5–3.0 cm. These findings contribute to defining effective guidelines for fiber-reinforced HLMs in conservation and structural rehabilitation.