Magnesium hydroxide was prepared under controlled conditions from aqueous Mg(NO3)2 and NaOH solutions. The small, nanoplate-shaped particle size distribution was monomodal from 164 to 459 nm. Functional polypropylene/Mg(OH)2 and polypropylene/polypropylene 1% maleic anhydride/Mg(OH)2 composites were prepared containing 10% or 30% Mg(OH)2. The composites have a high Young’s modulus (twice that of polypropylene) and comparable tensile strength but less ductility. EDX examination of the fractured composite surfaces suggested a homogeneous Mg(OH)2 distribution for composites produced with the addition of polypropylene grafted with maleic anhydride. The polypropylene/Mg(OH)2 composites showed good antibacterial activity. The polypropylene/polypropylene 1% maleic anhydride/Mg(OH)2 composites were less effective.