He was born in Bithynia of peasant stock. He worked as a swineherd, then became an officer in the Imperial army, where he served with such distinction in the war against the Bulgars that the Emperor Constantine VI wanted to take him into his personal service. "But the sight of massacres and horrors of war had brought home to him the vanity of this life. He asked leave of the Emperor to retire from the service, in order to wage unseen warfare in the ranks of the angelic army" (Synaxarion). In the coming years he traveled widely, sometimes living as a hermit, sometimes living in monasteries, more than once founding a monastic community. Wherever he went he lived in stillness, solitude and strict asceticism. He was famed for his spiritual counsel, his prophecies, his many miracles of healing ailments bodily and spiritual, and for his friendship with animals. Once a monk who doubted the Saint's miracles was eating at table with him when a large bear burst in upon them. Joannicius called the bear and it came and lay at his feet; he then told it to lie at the feet of his frightened guest and said "At their creation, the animals looked with veneration on man, who is made in the image of God, and he had no fear of them. We are afraid of them now because we have transgressed God's commandments. If we love the Lord Jesus and keep his commandments, no animal will be able to do us any harm." The monk departed greatly edified.
  In the last years of Joannicius' life, when he was about ninety years old, the Emperor Theophilus sought his counsel on the veneration of icons. The Saint's answer was pointed: "Whoever refuses due honor to the images of Christ, of the Mother of God and of the Saints, will not be received into the Kingdom of Heaven, even if he has lived an otherwise blameless life."
  Once Joannicius traveled to Constantinople to aid the Patriarch in some matters concerning the order of the Church. When he returned to his hermitage, he found that some jealous monks had set it on fire. Knowing who they were, he nevertheless addressed them kindly and invited them to share with him some food that he had managed to salvage from the fire. He did not attempt to rebuild his hermitage, but, taking the fire as a sign of his impending departure from this life, he traveled to the monastery of Antidion, where he had first entered into the monastic life and there, having predicted the day of his death, he reposed in peace. At the moment of his death, the monks of Mt Olympus saw a pillar of fire ascending from the earth to the sky.
  The Saint's relics have been the source of many miracles. His skull is kept and venerated at the Monastery of the Pantocrator on Mt Athos. The widely-used prayer "My hope is the Father; my refuge is the Son; my shelter is the Holy Spirit; O Holy Trinity, glory be to Thee!" is attributed to St Joannicius.