Recently, two-dimensional transient metal carbides and nitrides (known as MXenes) have attracted tremendous attention to develop electromagnetic interference (EMI) shield thin films, because of their large surface area and exceptional electrical conductivity. However, current technologies for developing MXene films are tedious, expensive, and unable to produce large films. Besides, MXene films are reported to suffer poor mechanical stability. Herein, we incorporated MXene into poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT: PSS) to develop thin films with high mechanical stability and EMI shielding. The ultrathin, uniform, and flexible MXene/PEDOT:PSS films were prepared by a simple drop-casting strategy at different MXene/(PEDOT:PSS) ratios. In particular, the nanocomposite film containing 25 wt.% PEDOT:PSS with a thickness of ~7 μm showed an exceptional electrical conductivity of 2,900±400 S cm-1 and an EMI shielding effectiveness of 55.4 dB after a co-treatment with sulfuric acid and methanol. Besides, the MXene/PEDOT:PSS nanocomposite films demonstrated exceptional mechanical stability by sustaining their electrical conductivity and EMI shielding properties following 500 bending cycles. The superior electrical conductivity, EMI shielding, mechanical stability, and flexibility of MXene/PEDOT: PSS nanocomposites suggest high potential of the developed technique to be adopted for the next generation of ultra-thin EMI shields.