The treatment of free-standing sheets of multiwalled carbon nanotubes (MWNTs) with a NH3/He plasma results in self-supporting sheets of aligned N-doped MWNTs (CNx). These CNx sheets can be easily twist spun in the solid state to provide strong CNx yarns that are knottable, weavable, and sewable. The CNx yarns exhibit tunable catalytic activity for electrochemically driven oxygen reduction reactions (ORR), as well as specific capacitances (up to 39 F.g-1) that are 2.6 times higher than for the parent MWNTs. Due to a high degree of nanotube alignment, the CNx yarns exhibit specific strengths (451 +/- 61 MPa.cm3.g-1) that are three times larger than observed for hybrid CNx/MWNT biscrolled yarns containing 70 wt.% CNx in the form of a powder. This difference in mechanical strength arises from substantial differences in yarn morphology, revealed by electron microscopy imaging of yarn cross- sections, as well as the absence of a significant strength contribution from CNx nanotubes in the biscrolled yarns. Finally, the chemical nature and abundance of the incorporated nitrogen within the CNx nanotubes is studied as function of plasma exposure and annealing processes using X-ray photoelectron spectroscopy and correlated with catalytic activity.