Constantine was crowned co-emperor by his father on 22 January 613 and shortly after was betrothed to his cousin, Gregoria, a daughter of his father’s first cousin, Nicetas. As they were second cousins, the marriage was technically incestuous, but this consideration must have been outweighed by the advantages of the match to the family as a whole. Furthermore, its illegality paled into insignificance beside Heraclius‘ marriage to his niece Martina the same year. In comparison, Constantine’s marriage was far less scandalous than that of his father’s.
Constantine and Gregoria married in 629 or perhaps early 630 and in that year their first child, Constans II was born. Their second child was another son, Theodosius. They also had a daughter named Manyanh who later married the last Sassanid King of Persia, Yazdgerd III.
Constantine became senior Emperor when his father died in 641. He reigned together with his younger half-brother Heraklonas, the son of Martina. His supporters feared action against him on the part of Martina and Heraklonas, and the treasurer Philagrius advised him to write to the army, informing them that he was dying and asking for their assistance in protecting the rights of his children. He also sent a vast sum of money, more than two million solidi, to distribute to the soldiers to persuade them to secure the succession for his sons after his death. Indeed, he died of tuberculosis after only four months, leaving Heraklonas sole emperor. A rumor that Martina had him poisoned led first to the imposition of Constans II as co-emperor and then to the deposition, mutilation, and banishment of Martina and her sons.