This study investigates effects of compositional variations and processing parameters such as ultrasonication time, carbon nanotube (CNT) content, and functionalization of CNT on the rheological, electrical, mechanical, and viscoelastic properties of epoxy-based composites filled with carbon nanomaterials. Expanded graphite (EG) was also used as a filler material into epoxy matrix to understand the effect of geometric features of carbon filler on the physical properties of epoxy. The effect of the void content and the agglomeration size with respect to CNT content, dispersion level, functionalization and the surface area on the composite properties has been investigated. Percolation threshold of CNT was found to be 0.1 wt% based on both rheological analysis and electrical conductivity measurements. High power and long-time ultrasonication have been found to have a detrimental effect on both the electrical and flexural properties due to breaking in CNT lengths. The electrical conductivity of composites was enhanced with the higher CNT loading but yielded bigger agglomerations. Computer tomography (CT) images were taken to visualize the void content of epoxy nanocomposites. ImageJ program was implemented to determine the void percentage from the CT images. Increase in resin viscosity resulted in higher void content in composites. Both the void content and the agglomerates possessed a negative impact on the mechanical and viscoelastic properties. EG-filled nanocomposites exhibited lower electrical conductivity than both the pristine CNT and functionalized CNT (f-CNT) reinforced epoxy nanocomposites. Three-point bending tests revealed that all type of nanoparticles (CNT, f-CNT, and EG) increased the flexural strength and strain compared to the neat epoxy.