According to Ammianus Marcellinus, Zosimus and Philostorgius, Justina was living near Sirmium by the time she was widowed. During the reign of Valentinian II, she moved with him to Mediolanum. She assisted her young son Valentinian II's rule. She was an Arian though unable to act in favor of her religious faction until after the death of her husband. She maintained a long struggle against Ambrose, leader of the Nicene faction in Milan, especially over the usage of basilica for an Easter celebration by the imperial court.