Flax fibers are suitable for lightweight applications and offer cost-effective solutions that do not compromise mechanical performance. Amongst natural fibers, flax fibers are the strongest and currently used in various mechanical and thermal-based applications. Chemical and structural compositions determine its behavior. Besides, many studies explore flax fiber surface treatments and various fabrication techniques to improve their performance. For instance, this compendious review explored the thermal behavior, water absorption, and durability of flax fiber-reinforced composites, including their hybridization and chemical treatments. Using such natural fibers helps improve the mechanical, thermal, and durability performances and enhances the environmental sustainability benefits. Thus, the natural fibers and their composites are easily attracted by various industries seeking to develop eco-friendly materials. In conclusion, this review concisely discusses flax fiber’s usefulness as a sustainable and cost-effective reinforcing element for polymer matrix composites.