During the First Jewish-Roman War (66-73), he ran into conflict with Josephus, the leading Jewish general in Gallilee. When the Romans had reconquered Galilee, Justus sought sanctuary with the Tetrach Agrippa. Vespasian, who led the Roman troops, demanded that Justus be put to death, but Agrippa spared him and merely imprisoned him. The tetrarch even appointed Justus as his secretary, but later dismissed him as unreliable.
Justus wrote a history of the war in which he blamed Josephus for the troubles of Galilee. He also portrayed his former master Agrippa in an unfavourable light, but did not publish the work until after Agrippa's death. Justus also wrote a chronicle of the Jewish people from Moses to Agrippa II. Both his works only survive in fragments.
Flavius Josephus, Justus' rival, criticized the Tiberian's account of the war and defended his own conduct in the Autobiography, from whose polemical passages we derive most of what we know about Justus' life.