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May-Thurner Syndrome (MTS) is an underrecognized anatomic cause of left-sided deep vein thrombosis (DVT) resulting from compression of the left common iliac vein by the overlying right common iliac artery. Although it accounts for approximately 2-5% of DVT cases, it may be overlooked when other provoking risk factors are present, leading to incomplete treatment and risk of recurrence. We report a 61-year-old woman with no prior history of venous thromboembolism who presented with one week of progressive left lower extremity swelling and pain following recent femoral venous catheterization during a prior hospitalization. Imaging demonstrated extensive proximal iliofemoral DVT involving the left external iliac, common femoral, and superficial femoral veins. Despite initiation of intravenous anticoagulation, the significant clot burden prompted aspiration mechanical thrombectomy. Post-procedural venography revealed persistent residual stenosis of the left common iliac vein, raising suspicion for underlying iliac vein compression. The distribution of thrombosis and focal stenosis was consistent with MTS. Balloon venoplasty was unsuccessful, and definitive management was achieved with placement of a 16 mm × 80 mm venous stent, resulting in restoration of inline venous flow. The patient was transitioned to oral anticoagulation with dual antiplatelet therapy and experienced clinical improvement without complications. This case highlights MTS as an important and potentially underdiagnosed cause of extensive left-sided DVT, particularly when thrombus burden appears disproportionate to apparent provoking factors. Recognition of this anatomic variant is essential, as anticoagulation alone may be insufficient and endovascular intervention is often required to prevent recurrence and long-term venous morbidity.