Background: Marine-Lenhart syndrome (MLS) is a rare thyroid disorder characterized by the coexistence of Graves' disease (GD) with autonomously functioning thyroid nodules. This overlap presents diagnostic and therapeutic challenges due to the dual hyperthyroid mechanisms involved. Case Presentation: A 32-year-old female presented with weight loss, palpitations, heat intolerance, and nervousness. Physical examination revealed a slightly enlarged thyroid without palpable nodules. Laboratory results confirmed hyperthyroidism with suppressed thyroid-stimulating hormone (TSH) and elevated free thyroxine (FT4) and free tri-iodothyronine (FT3). Thyroid scintigraphy identified a hyperfunctioning nodule in the right lobe, and ultrasonography confirmed a 2.5 × 2.0 cm hypoechoic nodule with increased vascularity. Fine needle aspiration cytology (FNAC) categorized the lesion as Bethesda category 2 (benign). Elevated TSH receptor antibody (TRAb) levels confirmed concurrent GD. The patient was diagnosed with MLS and initiated on carbimazole, achieving biochemical euthyroidism within three months. Given her personal circumstances, radioiodine therapy was deferred. Discussion: MLS remains underrecognized due to its variable presentation and resemblance to Plummer’s disease and classical GD. Accurate differentiation is critical, as the management strategy differs from isolated GD or toxic nodular goiter. While antithyroid drugs provide symptomatic control, definitive treatment often requires radioiodine therapy or surgery. The presence of thyroid nodules in GD also raises concerns about potential malignancy, necessitating close monitoring. Conclusion: This case highlights the importance of considering MLS in patients with Graves’ disease and coexisting thyroid nodules. A tailored approach integrating clinical, biochemical, and imaging findings is essential for optimal management.