Very high aspect ratio single-walled carbon nanotubes (SWCNTs), with lengths >10 μm, offer many advantages, in particular within the field of nanocomposite materials. However, the routine solution processing routes developed for typical ~1 μm SWCNTs do not disperse longer SWCNTs, instead damaging and cutting the nanotubes, producing tangled mats of dendritic agglomerates. Instead, a simple one-pot, reductive dissolution can be applied to these ultra-long SWCNTs to give solutions of negatively charged nanotubes in the >10 μm regime which are completely debundled to individualised species. The solutions of ‘nanotubide’ ions can be used in a new reactive coagulation spinning, utilising the inherent reactivity of the nanotubide to covalently attach matrix and nanotubes generating a strong interface during an in situ reaction. The fibres spun using solution-processed ultra-long SWCNTs show dramatic increases in strain to failure and toughness with no loss to strength or stiffness, relative to control fibres spun with other typical ~1μm commercial SWCNT sources.