The objective of present study is to prepare conductive polyimide (PI)–carbon nanofibers (CNFs) composite at very low percolation threshold. The combined advantage of in situ polymerization of PI in the presence of highly graphitized CNFs and continuous probe type sonication throughout the polymerization provide electrically conductive PI composites with a significantly lower percolation threshold than previously reported. The electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) of the nanocomposite with different weight percentages of CNFs (0.25–5 wt %) were investigated at room temperature. The measurement of EMI SE was carried out using thin film of composites in a frequency range of 8.2–12.4 GHz (X-band). The direct current conductivity of composites follows the percolation scaling law with a very low percolation threshold (CNF, 0.5 wt %). The electrical conductivity of the PI composite increases by more than 10 decades of magnitude, from 5.8 × 10−16 to 2.03 × 10−6 mho cm−1 owing to the addition of 0.5 wt % of CNF. An increase in dielectric permittivity and decrease in alternating current impedance with increasing concentration of CNF are observed. The conduction mechanism in composites is explained in the light of power law-dependent current (I)–voltage (V) characteristics. The composite of 0.07-mm thickness shows EMI SE of above 12 decibel (dB) at 5 wt % of CNF loading. The experimental data suggest that the prepared composites can be used for the dissipation of electrostatic charge and EMI shielding purpose. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40914.