Background <p>Obesity is a known risk factor for surgical complications, but its impact on mandibular reconstruction with fibula free flap (FFF) remains unclear. This study examines the association between body mass index (BMI) and postoperative morbidity.</p> Patients and Methods <p>We conducted a retrospective cohort study of patients undergoing mandibular reconstruction with FFF from 2000 to 2024. Patients were categorized by BMI: underweight (&lt; 18.5&#xa0;kg/m²), normal weight (18.5–24.9 kg/m²), overweight (25–29.9 kg/m²), and obese (≥ 30&#xa0;kg/m²). Primary outcomes included donor and recipient site complications; secondary outcomes included physical therapy data. Multivariate models adjusted for age, gender, smoking, comorbidities, and chemotherapy.</p> Results <p>Among 428 patients, 11.0% were underweight, 44.2% normal weight, 31.3% overweight, and 13.6% obese. Donor site complications occurred in 31%&#xa0;of patients, primarily delayed wound healing. Compared with normal weight patients (33% complication rate), underweight patients had significantly fewer donor site complications (11%, <i>p</i> = 0.020), while rates in overweight (30%, <i>p</i> = 1.0) and obese patients (45%, <i>p</i> = 0.358) did not significantly differ. Underweight status was independently associated with lower odds of donor site complications (OR 0.20, <i>p</i> = 0.002), and higher continuous BMI predicted increased donor site risk (OR 1.08, <i>p</i> = 0.001). BMI was not associated with recipient site complications or rehabilitation outcomes, although underweight patients experienced earlier recipient site complications on time-to-event analysis.</p> Conclusions <p>Higher BMI was independently associated with increased donor site morbidity, particularly in patients requiring skin graft closure. Underweight patients had fewer donor site complications but experienced earlier recipient site complications, underscoring the need for BMI-tailored perioperative strategies.</p>