Early life
After hearing of Saint Anthony, whose name "was in the mouth of all the races of Egypt" (Jerome), at the age of fifteen, he went to live with him in the desert for two months. As Anthony's hermitage was busy with visitors seeking cures for diseases or demonic affliction, he went home together with some monks. At Thabatha, his parents having died in the meantime, he gave his inheritance to his brothers and the poor and left for the wilderness.

Beginnings of Monastic Life
Hilarius went to the area southwest of Majoma, the port of Gaza, that was limited by the sea at one side and marshland on the other. It was the abode of robbers. With him he took only a shirt of coarse linen, a cloak of skins given to him by St. Anthony and a coarse blanket. He led a nomadic life, subsisting only on dried figs, eaten after sunset.
After he was beset by carnal thoughts, he reduced his diet to the juice of herbs and less figs. Cold showers not being available, he took to praying, singing, the hoeing of the soil and the production of baskets made from rushes. Although he was quite starved, "so wasted that his bones scarcely held together" (Jerome) he still had visions of naked women, voluptuous meals, chariots and gladiatorial contests. Often he heard voices, of infants or of domestic animals, which he identified as demons.
He finally built a hut of reeds and sedges, in which he lived for four years. Afterwards, he constructed a tiny low ceilinged cell, "a tomb rather than a house", where he slept on a bed of rushes, recited the bible or sang hymns.
He never washed his clothes, changed them only when they fell apart and shaved his hair only once a year. He was once visited by robbers, but they left him alone when they learned that he did not fear death (and had nothing worth stealing, anyway), promising to mend their ways.
Jerome gives a detailed account of his diet:
After that, he suffered from signs of malnutrition, his eye-sight grew poor, his body shrivelled and he developed dry mange and scabs, so he had to slightly modify his diet.
After he had lived in the wilderness for 22 years, he became quite famous in Palestine. Visitors started to come, begging for his help.

from 20-23: half a pint of lentils moistened with cold water
23-27: dry bread with salt and water
27-30: wild herbs and roots
31-35: six ounces of barley bread, and boiled vegetables without oil
35-63: six ounces of barley bread, and boiled vegetables with oil
63-80: six ounces of water, boiled vegetables with oil and a broth made from flour and crushed herbs, taken after sunset His Time at Majoma
His first miracle was when he cured a woman from Eleutheropolis who had been barren for 15 years. Later, he cured blindness, raised children from the dead, healed a paralysed charioteer, expelled demons. He even cured horses affected by evil magic and tamed a mad Bactrian Camel.

Miracles
In time, a monastery grew around his cell, which was so beset by visitors, especially females, that Hilarion fled.
After numerous adventures, always beset by enthusiastic visitors seeking his help, Hilarion died in Cyprus in 371 AD.

Sources
Hermann Hesse adapted a biography of St. Hilarion as one of the three Lives of Joseph Knecht', making up his Nobel Prize winning novel The Glass Bead Game (also known as Magister Ludi).