In the framework of an increasing global change towards sustainability aimed at the adoption of environmentally responsible practices, the construction industry is undergoing a transformative phase in redefining building materials. Traditional reinforcing materials present significant environmental concerns, necessitating the exploration of sustainable alternatives. One promising solution is the integration of natural fibers into mortars, offering both structural performance and ecological benefits. This study presents a comprehensive investigation into the use of plant-based fibers—namely jute, coconut, and hemp—as reinforcing materials of lime-based mortar. The selection of a lime matrix is particularly important for both sustainable construction and heritage conservation, as it provides benefits such as breathability and chemical compatibility with historic masonry. These fibers are selected due to their remarkable mechanical properties, including high tensile strength and flexibility, which can contribute to the enhancement of performances of traditional mortars. Hemp fibers demonstrate exceptional tensile strength, while their low environmental impact makes them ideal materials for sustainable retrofitting applications. Coconut fibers are recognized for their superior tensile strength and durability. Jute fibers, characterized by their long and strong structure, provide a balance of flexibility and load-bearing capacity. Firstly, this research examines the physical properties of these natural fibers, including water absorption capacity and workability of the resulting mortars. Additionally, mechanical characterization tests are conducted to evaluate the structural performance of fiber-reinforced mortars. Finally, the findings underscore the potential of these materials to enhance lime-based mortars, offering a lightweight and sustainable alternative for reinforcing plasters in seismic retrofitting applications.