Isolated external iliac artery aneurysm is a rare occurrence. These aneurysms have varied presentations depending on size and proximity. Both open surgical and endovascular modalities can be used for treatment depending upon presentation, aneurysmal anatomy, and patient condition. Preservation of at least one internal iliac artery is important to prevent post-repair hypogastric ischemia. There are no previous reports of IgG4-related disease (IgG4-RD) as etiology of these aneurysms. A 32-year-old male patient presented with a left lower abdominal lump and was found to have a left external iliac artery aneurysm on computed tomography angiography. The patient underwent iliofemoral bypass with an 8 mm polyester graft. Histopathological examination of the aneurysm wall suggested IgG4-RD. The patient fulfilled the 2020 Revised Comprehensive Diagnostic Criteria for IgG4-RD. An 18-Fluorodeoxyglucose-Positron Emission Tomography scan performed in the postoperative period showed no active disease, hence medical therapy was not instituted. The patient is doing well at 1 year.