A method of cutting, thinning, welding and chemically functionalizing multiwalled carbon nanotubes (CNTs) with carboxyl and allyl moieties, and altering the electrical properties of the CNT films by applying high current densities combined with air-exposure is developed and demonstrated. Such welded high-conductance CNT networks of functionalized CNTs could be useful for device and sensor applications, and may serve as high mechanical toughness mat fillers that are amenable to integration with nanocomposite matrices.