The monastery was founded in 613 and named after Gallus, an Irishman. Saint Gallus was a disciple and companion of Saint Columbanus. Gallus died in 646 in that location. Charles Martel placed Othmar there as custodian of St Gall's relics. During the reign of Pepin the short Othmar founded the famous schools of St. Gall, where arts, letters and sciences flourished. Copying manuscripts was undertaken and a famous library was gathered. Numerous Anglo-Saxon and Irish monks came to copy manuscripts. At Charlemagne's request Pope Adrian I sent distinguished chanters from Rome, who propagated the use of the Gregorian chant.
The library at St. Gall preserved one of the most famous surviving manuscripts of the middle ages, widely known as the "Plan of St. Gall". However, the monastery depicted in the Plan was not St. Gall, but an ideal of what a well-designed and well-supplied monastery should have as envisioned by one of the synods held at Aachen for the reform of monasticism in the Frankish empire during the early years of emperor Louis the pious (between 814 and 817).
St. Gall got into conflict with the nearby Abbey of Reichenau at Lake Constance. Between 924 and 933 Huns threatened the abbey and the books had to be removed to Reichenau for safety. Most were later returned.
Under abbot Pius (1630-1674) a printing press was started. In 1712 a great change came to St. Gall with the pillage by the Swiss. Most of the books and manuscripts were taken by them to Zürich and Bern. After confusion and suppression and rearrangements St. Gall became the residence of the bishop and held the offices of the Canton, along with holding the remains of the library.