Achieving stable dispersion of graphite flakes (GFs) in aqueous media remains a critical challenge in developing high-performance graphite-flake nanofluids (GNFs) for direct photothermal energy harvesting. This study explored the synergistic effects of anionic and non-ionic surfactants on colloidal stability, optical properties, and photothermal response of GNFs. Among all the engineered formulations, the mixed-surfactant system containing 20 wt.% Tween 80 (T80, non-ionic) and 80 wt.% sodium dodecyl sulfate (SDS, anionic), (GNF@T80₂₀%–SDS₈₀%) exhibited the highest dispersion stability with improved photothermal performance, maintaining uniform suspension for over 40 days. UV–Vis spectroscopy showed a distinct redshift in the π→π* transition, attributed to improved exfoliation and enhanced interfacial polarizability from the dual surfactant system, which stabilized the excited electronic states at lower energies. This optimized formulation achieved photothermal conversion efficiency of 91% under AM 1.5G solar irradiation, outperforming both single-surfactant and base fluid counterparts. Numerical simulations using a morphology-corrected Mie scattering model closely matched the experimental extinction spectra, validating the optical behavior. Radiative transfer analysis further confirmed ~93% total solar absorption before 3 cm depth. This work establishes a synergistic surfactant-assisted design and modeling strategy for producing stable, high-efficiency GNFs, paving the way for their application in next-generation solar thermal energy conversion systems.