Cervical thymic cysts are among the rarest congenital neck masses. They are probably more frequent than the number of cases reported as many of these lesions are asymptomatic and only discovered incidentally. The initial embryologic development of the thymus begins in the neck, followed by migration into the superior mediastinum. For this reason, extension of cervical thymic anomalies into the mediastinum is possible. Although it is the least common lateral cystic neck mass, it must be differentiated from other pediatric cystic neck masses, the majority of which are anomalies of the branchial system. Due to the possibility of mediastinal extension, the management of these lesions is different than other congenital neck masses. We report two cases of thymic anomalies with mediastinal extension and review the embryology, diagnosis, and management of cervicomediastinal thymic cysts.