Magnesium hydroxide nanoparticles are made from a magnesium compound that is reacted with an organic dispersing agent (e.g., a hydroxy acid) to form an intermediate magnesium compound. Magnesium hydroxide nanoparticles are formed from hydrolysis of the intermediate compound. The bonding between the organic dispersing agent and the magnesium during hydrolysis influences the size of the magnesium hydroxide nanoparticles formed therefrom. The magnesium hydroxide nanoparticles can be treated with an aliphatic compound (e.g., a monofunctional alcohol) to prevent aggregation of the nanoparticles during drying and/or to make the nanoparticles hydrophobic such that they can be evenly dispersed in a polymeric material. The magnesium hydroxide nanoparticles exhibit superior fire retarding properties in polymeric materials compared to known magnesium hydroxide particles.