He was born in Ireland to wealthy parents, who sent him to be educated at the Monastery of Bangor. There he embraced the ascetical life and became a monk. He was one of the twelve monks who traveled with his spiritual father St Columbanus (November 23) as missionaries to Gaul. In time some of the group traveled into pagan lands, up the Rhine river to Lake Zurich. The monks settled on Lake Constance around a chapel dedicated to St Aurelia, which had been taken by the pagans as a shrine; they cleansed and reconsecrated the chapel, which became the center of their new monastery. Saint Gall lived as a hermit, serving the brethren by making nets and catching fish. In 612 St Columbanus went on to Italy with most of his disciples, leaving St Gall and a few others to continue their life. When St Gall delivered Frideburga, the daughter of a local duke, from a demon, he offered the saint a tract of land on the shores of Lake Constance; here was founded the monastery that in later times bore St Gall's name.

At various times, the holy Gall refused calls to become a bishop, or to take over the abbacy of the great monastery at Luxeuil. To all such requests he answered that he would rather serve than command. He continued living in his isolated monastic community until he reposed in peace in 640, at the age of ninety-nine. In later years, and continuing well into the middle ages, the Monastery of St Gall became famed for the holiness of its monks and for its library.