The effects of carbon nanofiller geometry on the electrical percolation behavior, electromagnetic interference (EMI) attenuation, and EMI attenuation mechanisms of polymer composites were investigated. Polylactic acid (PLA) filled with carbon nanotubes (CNT), graphene nanoplatelets (GNP), or a hybrid mixture of CNT:GNP were prepared by melt mixing to formulate composites of random nanofiller dispersion and distribution states. The CNT-based composite showed higher EMI attenuation performance and electrical conductivity than the GNP-based and CNT:GNP-based composites. In addition, the CNT-based composite showed a higher ability to attenuate the EMI by absorption than the GNP-based and CNT:GNP-based composites. It is speculated that fiber-like geometry and the smaller surface area of CNT compared to GNP have reduced the reflection of the radiation at the composite external surface and facilitated the dissipation of radiation by absorption due to the effective 3D network within the PLA matrix.