The focus of this study is to determine the effect of the processing method on the mechanical, thermo-mechanical and electrical properties of graphite nanoplatelets (GNP) reinforced polyamide 12 (PA12) composites. The two processing methods studied are injection molding (IM) and selective laser sintering (SLS). The composites made by SLS exhibited higher tensile modulus, comparable or better flexural properties and lower impact strength than those made by IM. This enhancement is supported by the higher degree of crystallinity and more effective interfacial interactions observed in the SLS composites compared to those made by IM. At higher GNP content the property enhancement is compromised due to the presence of GNP agglomerates. Furthermore, it was found that the SLS composites filled with 5wt% of GNP exhibited four orders of magnitude higher electrical conductivity compared to their counterpart made by IM which exhibited electrical properties similar to the neat PA12. The understanding of the processing-structure-property relationship investigated here can lead to composites engineered for targeted applications.