One commonly practiced procedural step to reduce the risk of postoperative hematoma accumulation when performing cranioplasties is to place a closed negative-pressure subgaleal drain. We present a patient with sinking skin flap syndrome that underwent such a procedure and subsequently experienced immediate postoperative ascending transtentorial herniation and intracranial hemorrhage remote from the surgical site. On determining that the subgaleal drain was the responsible cause, it was immediate removed, and the patient had neurological recovery. Fewer than 30 cases of life-threatening subgaleal drain-related complications have been documented, and this is the first reported case of ascending herniation occurring after cranioplasty. This report illustrates the potential risks of subgaleal drainage, the importance of early recognition of this rare phenomenon and that intervention can be potentially life-saving.