The influence of morphology of carbon modifiers on the electrical, thermal, and mechanical properties of their polyethylene-matrix composites is reported. Four heat-treated (HT) carbon modifiers were investigated: PR-19-HT carbon nanofibers, multiwalled carbon nanotubes (MWNT-HT), helical multiwalled carbon nanotubes (HCNT-HT), and mesophase pitch-based P-55 carbon fibers as a control. These were melt-mixed with linear low density polyethylene at 10 vol %, which was above the percolation threshold. The electromagnetic shielding effectiveness (EM SE) of the composites exhibits significant dependence on the modifier morphology. Thus, MWHTs, with the highest aspect ratio, lead to the highest composite electrical and thermal conductivities (34 Sm−1 and 1 Wm−1 K−1) and EM SE (∼24 dB). In contrast, HCNT, due to their coiled shape and low aspect ratio, lead to segregated microstructure and low EM SE (<1 dB). However, these composites display the highest ductility (∼250%) and flexibility, probably due to matrix-modifier mechanical bonding provided by the helical morphology. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41055.