Background: Non-anastomotic pseudoaneurysm formation due to very late prosthetic graft failure after femoropopliteal bypass is exceptionally rare. Case Presentation: We describe a 73-year-old blind man who presented with rapid enlargement of a mid-thigh mass on the left side, associated with skin necrosis. His history included advanced atherosclerosis with bilateral superficial femoral artery occlusion and prior femoropopliteal bypasses: a right-sided great saphenous vein graft (2006) and a left-sided Dacron^® graft (2008). Computed tomography angiography revealed a giant pseudoaneurysm originating from the mid-portion of the left bypass graft (13.8 × 16.5 cm) with active contrast extravasation and distal popliteal artery occlusion, as well as a large, well-defined lateral thigh lipoma. Open surgery revealed structural graft disruption within the prosthetic body and a large chronic pseudoaneurysm sac containing organized thrombus. En bloc pseudoaneurysm excision and graft exclusion without reconstruction were performed, followed by soft-tissue reconstruction. The postoperative course was uneventful, with complete wound healing by four weeks and no ischemic symptoms during 18 months of follow-up. This exceptionally late prosthetic graft complication underscores the need for long-term surveillance in patients with lower-limb bypass grafts. Conclusions: This case highlights that prosthetic graft failure may occur very late and present insidiously. Recognition of this rare complication is essential for timely diagnosis and individualized surgical management.