Monteggia fracture is rare in infants and is frequently overlooked initially. This is especially true in cases that occur before the appearance of ossification centers in the elbow. Treatment should be optimized to avoid long-term complications. We report a 12-month-old child with neglected Monteggia fracture who achieved a successful outcome after receiving an arthrography-assisted closed reduction of the radial head dislocation and an ulnar osteotomy with plate fixation and bone graft. The patient had a stable radiocapitellar joint with full range of motion at the final follow-up.