Background The literature lacks consistent information on the correlation between baseline body mass index (BMI), clinical presentation, and prognosis in patients with myasthenia gravis (MG). This observational multicenter prospective cohort study included patients with MG from February 2017 to June 2023, categorizing them by baseline BMI. The primary outcome was the time to generalization of ocular MG. Secondary outcomes included the time to Activities of Daily Living (ADL) response and Minimal Symptom Expression (MSE). Kaplan-Meier curves and multivariable Cox proportional hazards regression models were used to assess the impact of BMI on these outcomes. Results Out of 940 MG patients (510 women) included, 524 had a low BMI and 416 had a high BMI, with a median age of 50.00 years. Patients in the high BMI group were significantly older (p < 0.001), had a lower percentage of females (p < 0.001), and had a shorter disease duration (p = 0.014) compared to those with a low BMI. They also had higher rates of ocular onset (p < 0.001), ocular MG classification (p = 0.001), and acetylcholine receptor antibody seropositivity (p = 0.007), but a lower incidence of thymectomy (p = 0.027). During a median follow-up of 33.00 months, the adjusted Cox models revealed that a higher baseline BMI was associated with an increased risk of ocular MG generalization (HR 1.06; 95% CI 1.01-1.11; p = 0.026), but not with ADL response (HR 0.99; 95% CI 0.95-1.04; p = 0.779) or MSE (HR 0.97; 95% CI 0.92-1.02; p = 0.240). Conclusions A higher baseline BMI was associated with an increased risk of ocular MG generalization but not with ADL response or MSE.