We demonstrate that high current densities, combined with air exposure can slice, weld, and chemically functionalize multiwalled carbon nanotubes (CNTs) with carboxyl and allyl moieties, and alter the electrical properties. The conductance of thin film assemblies of CNTs increases by 150%, indicating that the increase in the number of low-resistance pathways caused by CNT junction formation offsets the conductance decrease expected from defect creation, surface functionalization, and fissure. 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.