Introduction: Fibrous dysplasia (FD) is a bone disorder in which fibrous tissues replace the natural bone matrix. FD can affect any bone in the body; however, it most commonly affects flat and long bones. Aneurysmal bone cysts (ABCs), often described as “blood-filled cysts,” are a benign (non-cancerous) type of bone disease. The tibia and femur are among the bones most commonly affected by ABCs. Concurrent FD with secondary ABC in long bones is very rare. Case Report: We present a case of a 9-year-old girl, who presented to the emergency with pain in her left groin and an inability to bear weight on her left leg. Complete case history was recorded and a preliminary medical examination was performed. Based on radiological findings and non-contrast computed tomography scans, the subject was diagnosed with FD and a concurrent bone cyst. The MRI showed a large fluid–fluid level in the proximal part of the lesion, indicating FD with ABC. Surgical curettage and debridement, followed by plating and bone grafting were planned. Post-operation biopsy performed on the curetted materials confirmed concurrent FD with ABC. Discussion: Concurrent FD with ABC is rare. After through literature review, we could identify a few reported cases of concurrent FD with ABC in long bones. Conclusion: Concurrent FD with ABC-like changes is best visualized on MRI, which can reveal conventional fluid–fluid levels, aiding in accurate diagnosis and treatment planning.