Possibly a relative of the Constantinian dynasty (), she was Valentinian's second wife after Marina Severa, and step-mother of the "augustus" Gratian () and the mother-in-law of the "augustus" Theodosius I (). Her infant son Valentinian was made emperor shortly after her husband's death in November 375. According to Late Antique ecclesiastical history, Justina was an Arian Christian, and began to promote this christology after her husband died, bringing her into conflict with Ambrose, the Nicene Christian bishop of Mediolanum (Milan). In 387, fleeing from the invasion of the Italian Peninsula by the rebel "augustus" Magnus Maximus (), Justina took her children to the Balkans – including the child-emperor Valentinian II – and secured the intervention of the eastern "augustus" Theodosius in the civil war by marrying her daughter Galla to him at Thessaloniki. Afterwards, Theodosius attacked and defeated Magnus Maximus, ending the civil war, during which time Justina herself apparently died.