The case describes a 17-year-old male with chronic right hip pain and a 3-cm limb-length discrepancy following a prior femoral neck fracture from a motor vehicle crash that progressed to nonunion and was treated with a valgus-producing proximal femoral osteotomy. He later developed traumatic avascular necrosis with global femoral head collapse, making hip preservation options nonviable. Given persistent symptoms and complex altered anatomy from prior surgeries, he underwent conversion to total hip arthroplasty using careful preoperative templating and a 3D-printed femoral model to optimize implant selection and alignment. Postoperatively, he achieved good pain relief and functional recovery, though residual limb-length discrepancy required staged contralateral epiphysiodesis due to remaining growth potential. The case highlights that, while uncommon in adolescents, total hip arthroplasty can provide meaningful improvement in pain and quality of life in carefully selected young patients with end-stage hip pathology, with special consideration needed for growth, anatomy, and long-term implant survivorship.