This work presents an ultrasonic-assisted approach for fast impregnation of thermoset and thermoplastic polymers including solvent-free epoxy resins, polydimethylsiloxane resins, and polypropylene into multi-walled carbon nanotube (MWCNT) nanopapers to prepare nanocomposites or prepregs with high loading (> 20 wt%) of well-dispersed MWCNTs, not achievable by the conventional compounding or resin transfer molding methods. The approach involves making MWCNTs into macroscopic paper-like sheets (also called "nanopapers") via a dispersion-filtration method and then impregnating the mesoporous nanopapers with polymeric resins using an ultrasonic infiltration method to form a prepreg or nanocomposite. The formed nanocomposites revealed an excellent combination of mechanical strength, sand erosion and scratch resistance, electrical and thermal properties, and good processing flexibility.