Reaction of a Mg-Al carbonate layered double hydroxide (LDH) with boric acid leads to a borate-pillared LDH with the stoichiometry [Mg0.65Al0.35(OH)2][B3O5]0.35·0.65H2O and an interlayer spacing of 1.07 nm. Infrared and 11B magic angle spinning nuclear magnetic resonance data are consistent with the presence of polymeric triborate anions of the type [B3O4(OH)2]nn− in the interlayer galleries so that the material can be formulated as [Mg0.65Al0.35(OH)2][B3O4(OH)2]0.35·0.30H2O. The flame-retardant properties of the borate-pillared material and the carbonate precursor in composites with ethylene vinyl acetate copolymer were compared. Introduction of the borate anion leads to a significant enhancement in smoke suppression during combustion without compromising the flammability of the material. This is related to the synergistic effect between the host layers of the LDH and the borate anions uniformly distributed in the interlayer region.