Amid growing environmental concerns, manufacturing organizations are increasingly turning to Lean Six Sigma practices to drive sustainability. This study proposes an assessment model designed to evaluate the degree to which lean manufacturing practices (LMP) are used to improve environmental performance. Building on a systematic literature review, the model integrates sixteen LMPs with ten environmental performance indicators to establish a structured mapping framework. The model was applied in a case study within the oil and gas sector, analyzing seventeen Lean Six Sigma projects aimed explicitly at environmental improvement. Through this assessment, the company was able to recognize the strong application of key practices and identify areas where other LMPs could be further leveraged to broaden environmental impact. The resulting model not only quantifies the relationship between specific lean practices and environmental indicators but also acts as a diagnostic tool, supporting organizations in refining their sustainability strategies. This research contributes a practical and scalable methodology for evaluating and guiding lean-based environmental initiatives in industrial settings.