Lemongrass fiber was analyzed to determine the chemical proportion of its lignocellulosic components. Fibers’ thermal behavior, surface structures, and functionality were assessed by thermogravimetric analysis (TGA), scanning electron microscope (SEM), and Fourier transform-infrared spectroscopy (FT-IR), respectively. High-density polyethylene (HDPE) matrix composites filled with varying (10%, 20%, 30%, 40%, and 50%) fiber content were prepared and investigated. Composite wicker was made from HDPE and low density polyethylene (LDPE) blend-matrix and 10% alkaline modified fiber. Alkaline or maleic anhydride grafted polypropylene (MA-g-PP) was used to improve the compatibility between the fiber and matrices. The composites were evaluated by using TGA, SEM microscopy, and universal testing machine, respectively. The fiber was constituted by equitable amounts of lignocellulosic components with cellulose accounting for the highest proportion. It also exhibited high degradation temperature, which was further increased following alkaline modification. Superior thermal degradation behavior was measured for modified fiber composites. SEM showed that the modified fiber composites demonstrated better compatibility. Lemongrass fiber reinforcement substantially improved the mechanical properties of the composites.