Born in 103, he was a philosopher from the Samaritan town of Shechem in Palestine, who had devoted his life to the search for truth, trying many philosophical schools and sources of human wisdom: the Stoics, the Peripatetics, the Pythagoreans and finally the Platonists. One day an old man (whose name and origin are unknown) appeared to him and spoke to him of the Prophets and Apostles who had learned of God not by their own wisdom, but by revelation of God Himself. He read the scriptures and was convinced of the truth of the Faith, but he would not be baptised or call himself a Christian until he had tested all the pagans' arguments against Christianity. To this end he traveled to Rome, where he engaged in debate at philosophical gatherings, impressing all with his wisdom. In Rome he also witnessed the martyrdom of Sts Ptolemy and Lucian; this moved him to write an Apologia for the Christian faith and the Christian people, which he gave to the Emperor Antoninus and the Senate. They were so moved by this document that the Emperor ordered that persecution of Christians should cease.

For the remainder of his life, Justin devoted all his skills to the proclamation of the Gospel and the defense of Christians. To the end of his life, wherever he preached Christ, he always wore his philosopher's garb. In addition to his Apologia, he wrote a number of other learned defenses of the faith.

Eventually he was imprisoned following the false accusations of Crescens, a jealous Cynic philosopher. He died (one source says by beheading, another by poison) in Rome in 167 under the Emperor Marcus Aurelius, successor to Antoninus.