According to Ammianus Marcellinus, her mother was Faustina and her father was Constantius II, who died before Constantia was born.

On 26 June 363, Julian was fatally wounded in the Battle of Samarra against the forces of Shapur II of the Sassanid Empire.

Faustina and her young daughter's presence suggested that Procopius was the rightful heir of the Constantinian dynasty which was still held in reverence.

Ammianus Marcellinus tells that Procopius "always bore with him on a litter the little daughter of Constantius and grand daughter of the great Constantine, with her mother Faustina, both when marching and when preparing for battle, thus exciting the soldiers to fight more resolutely for the imperial family, with which, as he told them, he himself was connected.

In 374, Constantia, who was about twelve years old, was just reaching marriageable age when she was sent west to marry Gratian, who was about fourteen and was the eldest son and co-ruler of Valentinian I.

[11] On 27 June 374, the dedication of a bath complex in Calabria first mentions Constantia as an empress alongside her stepmother-in-law Justina.