Flame retardant, environmentally sustainable nanocomposites were made by melt blending poly (lactic acid) (PLA) with melamine polyphosphate (MPP) and Cloisite 30B (C-30B). The composition of the nanocomposite was highly specific and guided by interfacial energy minimization principals which balanced enthalpic and mechanical contributions. In this critical range, even small changes in the filler concentration can have a large impact on the performance of the material. We showed that while addition of 17% MPP can increase the flame resistance of PLA, (achieving a UL-94 V2 rating) the mechanical properties were significantly degraded. Addition of only 1% C-30B reversed these effects and yielded a nanocomposite with enhanced mechanical properties and passing the UL-94 V0 flame test. This compound was able to be extruded and fed into a Makerbot Replicator 2X Fused Deposition Modeling (FDM) 3D printer, where the printed samples were indistinguishable mechanically from their molded counterparts and also achieved the UL-94 V0 rating. The enhanced performance occurred only within a very narrow composition window. Cone calorimetry revealed that while a significant decrease in the heat release rate, accompanied by the formation of an intumescent char, was achieved by the addition of 1% C-30B to the MPP/PLA blend, a very poor char, corresponding the increase in the heat release rate, was obtained with addition of 2% C-30B. These results were explained in view of the altered phase morphology observed in Transmission Electron Microscopy (TEM).