Natural nanocomposite materials such as nacre and chitin are known for their exceptional mechanical properties that combine diametric properties of strength and toughness. These property enhancements are largely credited to the adhesive binding role of the polymer matrix that intercalates the inorganic minerals, making the polymer adhesive a crucial element in the design of bio-inspired structural nanocomposites. One of the most well-known natural adhesive polymers is the mussel adhesive protein, which contains an abundance of catecholic amino acids such as 3,4-dihydroxyphenyl-L-alanine (DOPA) that show remarkable adhesion to inorganic surfaces in underwater environments. In this work, we demonstrate that these mussel-inspired ligands (MILs) provide an excellent chemical platform to grow stabilize inorganic minerals in situ, and confer exceptional mechanical reinforcement in hydrated nanocomposite materials. Using nitrodopamine-functionalized poly(ethylene glycol) gels, we show that MILs can serve as scaffolds for ¬in situ nucleation and growth of inorganic minerals such as magnetite in a spatially well-dispersed manner throughout the network. We show that these fillers serve as mechanically effective multi-functional cross-link sites, causing a 400 x increase in the modulus of the hydrated material with just 2.5 wt. % of magnetite. We demonstrate the generality of our strategy, and show similarly effective property improvements in other polymeric systems containing MILs such as dopamine-functionalized poly(ethylene glycol) and pyrogallol-functionalized poly(allylamine). Our work provides an introductory roadmap for adopting MIL-based polymeric adhesives as a new motif for the design of robust hydrated nanocomposite materials.