This research work aims to study the flexural and compressive behavior of esparto and jute natural fiber–reinforced composites and to investigate the influence of certain critical factors, in particular the nature, length, and ratio of fibers introduced in the cementitious matrix. The mortar matrix has a fragile character, whereas fiber-based mortar exhibits ductile behavior. The incorporation of the fibers avoids brittle breakage of the composite material and induces a more resilient behavior. The breaking energy of all fiber-reinforced mortars is significantly higher than that of non-reinforced cement mortar. This result is very interesting and clearly shows the interest of using these fibers in the new composites proposed. On the other hand, a comparison of the mechanical properties of these new formulated materials with those reinforced by glass fibers was conducted. Sustainable natural fibers outperform glass fibers in composite applications, delivering superior fracture energy absorption. Therefore, the biodegradable natural fibers employed in this study have a promising potential to replace synthetic fibers characterized by a high carbon footprint. Thus, thanks to these fibers, we can produce sustainable and eco-friendly products while ensuring very good mechanical properties. In fact, the response of fiber-reinforced mortar to compressive test reveals that for a constant fiber rate, the compressive strength increases proportionally with the fiber length, whereas, for a constant fiber length, the compressive strength increases when fiber content passes from 1 to 2% and then it decreases at 3%. The bending resistance reaches its maximum (2.83 MPa) for an esparto fiber length equal to 10 mm, while a length of 25 mm of glass fiber leads to a flexural resistance equal to 2.055 MPa. Compressive strain value is maximum (2.8 MPa) for a fiber ratio equal to 2% case of esparto fiber and decreases when fibers are added up to 3%. The same result was obtained for mortars reinforced with jute fibers. A detailed statistical investigation thanks to ANOVA tool was conducted in order to determine the effect of parameters on composite mechanical performances. This research proved that the most influencing parameters on the flexural and compressive resistance properties are nature and length of fibers. Concerning compressive modulus, it was shown that fiber ratio is the most impacting parameter. In addition, a process optimization was proposed. Moreover, laws of evolution of the physical parameters especially the elastic modulus and the strain at peak stress were suggested via an analytical model so as to simulate the response of mortars reinforced by natural reinforcements. The main result of this research showed the promising potential of using esparto and jute fibers to produce sustainable construction materials with good mechanical properties and low cost.