The current study focuses on the influence of low nano-carbon loading in polymer based composite fibers to modify matrix microstructure. With regards to the processing–structure–property relationship, post-spinning heat treatments (i.e., drawing, annealing without tension, and annealing with tension) was used to track microstructural development and associated mechanical property changes. Drawing and annealing procedures were found to influence the interphase volume fraction, fibril dimensions, sub-fibrillar lamellae, and sub-lamellae grain size for each sample. Annealing at 160 °C was found to have the largest impact on interphase percentage, fibril length, and grain packing density. These improvements corresponded to excellent mechanical properties for both control and composite fibers. Understanding the relationship between processing and property provides a novel perspective for producing high-performance composite materials from flexible polymers by only minimal amounts of carbon nano-fillers.