Background: Cavernous lymphangiomas are rare benign lymphatic tumours in the head and neck region. Cervicomediastinal extension of cavernous lymphangiomas are exceedingly rare in adults. This is probably the first published report of adult cervicomediastinal cavernous lymphangioma in India to our knowledge, highlighting its clinical presentation, diagnostic process, and management. Case Presentation: This case report describes a 20-year-old male with a painless neck swelling found intraoperatively to be a cervicomediastinal cystic mass, which was surgically excised and confirmed as a cavernous lymphangioma on histopathology. Conclusion: This case underscores the importance of considering cervicomediastinal cavernous lymphangioma in young adults presenting with neck masses. Early diagnosis and surgical intervention are crucial to prevent complications associated with mass effect on adjacent structures. The patient remains asymptomatic with no recurrence at follow-up, emphasizing the effectiveness of surgical management in such cases.