The conservation of the architectural heritage is a primary concern for the preservation of the cultural identity of a community. To this end, the choice of appropriate materials focusing on the economic and ecological compatibility with the existent ones is the main objective of scientific research in building materials for restoration and strengthening activities. Lime mortar is one of the main components of the heritage architecture, used as joints between bricks or stones, plasters or binders inside multi-leaf walls and in most cases is the component most prone to degradation. During the Roman era, above all, there were several improvements in theories, techniques and additives to be used in the mortar mix. Vitruvio was one of the most important architecture’s theorist and he proposed several recipes to be followed for different situations in order to obtain specific mechanical and physical properties. This study explores a novel biocomposite material compatible with the structural strengthening and restoration of ancient masonry buildings constructed with lime mortar, following a recipe attributed to Vitruvius. The mortar was mixed with Spanish broom and hemp fibers. The first phase involved mechanical characterization through three-point bending and uniaxial compression tests. The second phase assessed the effects of thermal and humidity-induced stress on the mixture and its mechanical properties.