Carbon fibers (CFs), renowned for their remarkable properties, has broad relevance across diverse sectors, as a reinforcing material in composites, not limited to the aerospace, automotive and the renewable energy sectors [1]. However, the inherent chemical inertness of the fiber surface limits the full reinforcement potential of CF in composites [2]. As a result, extensive research has focused on improving the fiber-matrix adhesion via surface modification strategies and is a key focus of ongoing research [3]. While most reported surface modification strategies translate well to pristine CFs, few are applicable to recycled CFs, making the transition of the latter to landfills the more convenient option. Hence, an on-demand and scalable thermal surface modification was developed on CFs, which can be readily translatable to recycled CFs. This research employs carbenes, generated via thermal activation of bisdiazomethanes, which are known to spontaneously tether onto substrates via bond insertion reactions via C-H and X-H (where X=O, N, and S) or bond addition reactions via C=C bonds [4]. Initially, pristine CFs were subjected to surface modification via immersion in bisdiazomethane solutions of 1,5, and 10 mmol concentrations, followed by air-drying and a thermal activation at 120 °C. The treated fibers retained their tensile strength and Young’s Modulus, with respect to the controls, while exhibiting a significant improvement in interfacial shear strength (IFSS). The highest improvement was recorded at 189% (5 mmol treated fibers) with notable increases of 54% and 97% recorded for the 1 and 10 mmol treated fibers respectively. Importantly, the IFSS improvements were maintained when the process was adapted for integration into an existing CF manufacturing line (30-second dip, 2-minute heating), resulting in 74-79% improvements. The second part of this work focused on the translation of this strategy onto recycled milled CF, via a spray coating process, using an industrially common solvent, acetone. Addition of these fibers to an epoxy polymer at wt% of 0.5, 1, 2, and 5 wt%, revealed statistically significant improvements in mechanical properties. The highest improvements were recorded at 15.37 % (2 wt%), 10.21 % (5 wt%), 14.63% (5 wt%), and 17.36% (1 wt%) for tensile strength, tensile modulus, flexural strength, and flexural modulus, respectively. Hence, this thermal surface modification, while serving to improve the mechanical properties of pristine fibers, also takes a step further to improve properties of recycled fibers enabling high-value second life applications.