This study investigates the sustainable synthesis of biobased graphene (BG) derived from coconut husk waste and its application in eco-friendly water-based drilling muds (WBM). The BG was prepared through thermal exfoliation of lignin and utilized as a fluid loss additive, while benzimidazole (BI) was incorporated to serve as a corrosion inhibitor. To optimize performance, the Taguchi method was combined with Grey Relational Analysis (GRA), targeting three key parameters: viscosity, fluid loss, and corrosion resistance. Structural characterization revealed that BG synthesized at 1000 °C exhibited improved graphitic ordering, with an average flake diameter of around 20 nm and an interlayer spacing (d-spacing) of 3.49 Å. In terms of performance, incorporating 0.5 wt% BG reduced fluid loss by 50%, while 5 wt% BI delivered an impressive corrosion inhibition efficiency of 96.9%. The optimal mud formulation was achieved using 0.5 wt% BG, 5 wt% BI, 60 min of mixing time, and 8 wt% bentonite. Altogether, this work highlights a sustainable pathway for drilling fluid formulation by valorizing agricultural waste and minimizing additive loadings—without compromising on performance or environmental compatibility.