The invention provides a submicron magnesium hydroxide particulate composition comprising a first distribution of magnesium hydroxide particles having a D50 of no more than about 0.30 .mu.m, a D90 of no more than about 1.5 .mu.m, and a BET surface area of at least about 35 m.sup.2/g, which can be used as a flame-retardant additive for synthetic polymers, optionally in combination with other flame-retardant additives such as nanoclays and larger-sized magnesium hydroxide particulate compositions. Polymeric resins comprising the submicron magnesium hydroxide particles and methods of manufacturing submicron magnesium hydroxide particles are also provided.