Phosphorus (P) is a critical macronutrient for plant growth, yet its availability in soil is often limited due to fixation in insoluble forms. The application of magnesium nanoparticles (MgNPs) has emerged as a promising strategy to enhance phosphorus mobilization by stimulating beneficial enzymatic activity in both plant and fungal systems. This study investigates the impact of MgNPs on the secretion of phosphorus-mobilizing enzymes—particularly acid phosphatase and phytase—in five economically significant crops: cluster bean (Cyamopsis tetragonoloba), mung bean (Vigna radiata), moth bean (Vigna aconitifolia), pearl millet (Pennisetum glaucum), and sorghum (Sorghum bicolor), as well as in selected fungal species including Aspergillus niger, Trichoderma harzianum, and Penicillium chrysogenum. Application of MgNPs under controlled conditions led to a significant increase in enzymatic activities associated with phosphorus solubilization. Treated plants exhibited enhanced growth metrics—such as root development and chlorophyll content—coupled with elevated secretion of acid phosphatase and phytase enzymes in the rhizosphere. Similarly, fungi showed intensified enzyme production in response to MgNP exposure, indicating improved mineralization of organic phosphorus sources. These findings demonstrate that MgNPs significantly enhance phosphorus-mobilizing enzyme activity in both plants and fungi, thereby offering a novel approach to improving phosphorus bioavailability in agricultural soils. This work supports the potential of MgNPs as an efficient, nano-enabled tool for sustainable nutrient management, with future studies warranted to explore underlying gene regulatory mechanisms and field-scale applications.