Recently, lignocellulosic biomass-based plastics have attracted attention as alternatives to oil-derived plastics. However, the conventional production of biomass-based plastics is a multistep process, requiring harsh pretreatment, chemical modification, and purification, resulting in high costs and environmental burden. Herein, we report the direct conversion of lignocellulosic agricultural waste, sugarcane bagasse, into an injection-moldable cellulosic thermoplastic having good mechanical properties. Specifically, the hydroxy groups in bagasse were substituted by long-/short-chain mixed acyl groups (20:80 molar ratio of decanoyl/acetyl groups) in a one-pot, two-step homogeneous transesterification reaction using the corresponding vinyl esters and an ionic liquid (1-ethyl-3-methylimidazolium acetate) as both solvent and catalyst. The lignin component in the bagasse derivative was separated by precipitation in methanol, and a mixed-ester derivative of cellulose and hemicellulose (polysaccharide acetate decanoate, PSAD) having long- and short- acyl chains was obtained. PSAD could be injection-molded at 205 °C because of the plasticizing effects of the decanoyl groups and the hemicellulose component, despite the ultrahigh weight-average molecular weight of >1.5 × 106 g mol–1. The hemicellulose derivative in PSAD yielded moderate flexibility and sufficiently high mechanical strength for its use as a thermoplastic with tensile and flexural strengths of 50 and 80 MPa, respectively.