In 618, Jonas arrived at the monastery of Bobbio Abbey in the province of Pavia, just three years after the death of its founder Columbanus, and he asserted that he had based his account of the great Irish saint on the testimony of persons who had known him intimately. "As is usual in such biographies, the miracles are numerous; for the contemporaries these formed the most valuable portions; for modern students they are full of instruction, and throw much light on the daily life of the monks" (Munro 1895). The monastery was a center of Early Medieval learning, supported by a library that was outstanding in its day. Jonas was appointed to a position of confidence, probably that of secretary to the abbots Attala (died 627) and to his successor Bertulf, whom Jonas accompanied on a journey to Rome in 628. Immediately after his return he moved to Gaul, for his life of Eustace, abbot of Luxeuil, (died 629), reflects personal acquaintance.
Appealed to by Saint Amand for assistance in his missionary work among the pagans of what is now Belgium and northern France, which occasioned his vita of Saint Vedast or Vaast, the first Frankish Bishop of Arras. In fulfilment of a promise made to the Black Monks of Bobbio during a short return visit to the monastery in 639, he wrote between 640 and 643 his principal work, the Life of St. Columbanus.
In 659, when he was sent by the Queen-Regent Balthild on a mission to Châlon-sur-Saône, he was referred to as "abbot", though of which monastery it cannot now be determined. During this journey he sojourned for a few days at the monastery of St. John of Réomé (Reomans, now Moustier-Saint-Jean) in the diocese of Langres. To comply with a request made by the monks on this occasion he wrote the life of their founder.
The other works of Jonas are lives of the abbots Attala and Bertulf of Bobbio, of abbot Eustace of Luxeuil, an abbey founded by Columbanus that retained close personal ties with Bobbio, and of the abbess Burgundofara (or Fara) of Evoriac (modern Faremoutiers).
Eustace, Attala, and Bertulf, he knew personally. Bede incorporated these lives into his Ecclesiastical History, while Flodoard turned that of Saint Columbanus into hexameter verse. The "Life of St. Fara" is chiefly an account of miraculous events alleged to have occurred during this saint's rule at Evoriac, but Jonas' elaborate and fantastically miraculous account contains nuggets of historical information that throw precious light upon a poorly-documented time.
The works of Jonas, exclusive of the "Life of St. Vaast," are printed in Patrologia Latina LXXXVII, 1011-88; a better edition by Krusch is in Monumenta Germaniae Historica: Script. Rer. Mer., III, 406-13, 505-17; IV, 61-152 (Hannover, 1896 and 1902).