Styrene acrylate polymer (SAC) nanocomposites with various carbon nanofillers (multi-walled carbon nanotubes, MWCNT, and onion like carbons, OLCs, annealed at two different temperatures) are manufactured by means of latex based and combined routes. Concentration of the carbon nanofillers is changed in a broad interval starting from 0.01 up to 10 wt%. Elastic, dielectric, and electromagnetic interference (EMI) shielding efficiency of SAC nanocomposites are investigated. Dynamic elastic modulus, electrical conductivity, and EMI shielding efficiency of the investigated SAC nanocomposites increase along with rising nanofiller content. The effect of the addition of anisodiametric MWCNT on the elastic properties as well as EMI shielding characteristics of the SAC nanocomposites is higher than in the case of the addition of OLCs. Modification effectiveness of the polymer matrix in respects to elastic and EMI shielding performance provided by the OLC, annealed at higher temperature, is greater in comparison with the OLC, annealed at lower temperature, generally because of more perfect structure of the first.