Nothing is known of Comentiolus's early life, except that he hailed from Thrace. He first appears in 583, as an officer (scribon) in the Excubitores, the imperial bodyguard, when he accompanied a Byzantine embassy to Bayan I (r. 562–602), the khagan of the Avars. According to the historian Theophylact Simocatta, he enraged the khagan with an outspoken statement, and was briefly imprisoned.