The olive oil activity generates annually a huge amount of biomass waste from the pruning action of the tree and fruit harvesting, including thin woody branches and leaves. In an effort toward more sustainability in the lignocellulosic biorefinery of olive waste activity, nanocelluloses (NCs) were produced from the solid residues remaining after the extraction of biological active compounds from the leaves. Fractions containing cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and lignin-containing cellulose nanofibrils (LCNFs) were prepared and characterized in terms of morphology, chemical composition, thermal stability, colloidal properties, and crystallinity. The reinforcing capability of the different fractions was evaluated in nanocomposite films made of an acrylic matrix incorporating different NC contents. Dynamic mechanical analysis (DMA) and tensile testing of the films were used to compare the reinforcing effect of the NCs. Although CNCs displayed the highest reinforcing effect based on the weight content, LCNFs and CNFs demonstrated also a reinforcing effect, which is higher when normalized with the cellulose content. Compared to CNFs and CNCs, LCNFs displayed a lower hydrophilic character confirmed by contact angle measurement, and had better thermal stability. This work demonstrated the potential use of the solid waste from olive leaves as a new source for biobased nanoparticles containing lignin, hemicellulose components, and cellulose nanofibrils. Beside the high content in bioactive polyphenol compounds that can be extracted from olive leaves, the conversion of the residual biomass can contribute to increasing the circularity and sustainability of olive oil activity.