This work investigated the effect of incorporating the alkali-treated Phoenix sp. fibers (PSFs) and nano-clay (NC) on the physico-mechanical, water, and thermal resistance properties of geopolymers composites. These composites were produced using low calcium fly ash-based geopolymer matrix and PSFs and NC as a reinforcing component. Findings showed that incorporating the alkali-treated fiber and NC significantly enhanced the mechanical characteristics of the composites compared to the control matrix and untreated fiber composites. Specifically, the composite containing 2 wt.% of NC improved their compressive strength by 40.53%, splitting tensile strength by 195.31%, flexural strength by 185.32%, and fracture toughness by 71.93% compared to the control matrix. Further, composites containing treated fiber and NC exhibit a significant reduction in water absorption (30.18%) and thermal conductivity (12.5%). An organised matrix-fiber interface was revealed by scanning electron microscopy (SEM) images, this elucidated the favourable mechanical qualities that were attained. Furthermore, load–deflection tests were performed to determine the viability of fly-ash-based geopolymer composites (FAGC) for structural applications. The findings indicate that reinforcing the alkali-treated PSFs and NC improves the overall performance of the FAGC. More importantly, this research promotes the sustainable use of agricultural and industrial wastes, as well as sustainable development.