Born Christopher Fleming, his father was great-grandson of Christopher Fleming, 8th Baron Slane; his mother was a daughter of Robert Cusack, a Baron of the Exchequer and a close relative of Lord Christopher Nugent. In 1612 Fleming went to Flanders, and became a student, first at St Patrick College of Douai (university of Douai), and then at the College of St. Anthony of Padua at Leuven (French: Louvain).
In 1617 Fleming entered the Order of Friars Minor, and assumed the religious name of Patrick. A year later he made his solemn profession of religious vows. Five years after his, he went to Rome with Hugh MacCaghwell, the Definitor General of the Order, and when he had completed his studies at the College of St. Isidore, was ordained a priest.
From Rome Fleming was sent by his superiors to Leuven and for some years lectured there on philosophy. During that time he established a reputation for scholarship and administrative capacity, and when the Franciscans of the Strict Observance, the branch to which he belonged, opened a college in Prague, Fleming was appointed its first Guardian. He was also named a lecturer in theology.
The Thirty Years' War was raging at this time, and in 1631 the Elector of Saxony invaded Bohemia and threatened Prague. Fleming, accompanied by a fellow-countryman named Matthew Hoar, fled from the city. On 7 November the fugitives encountered a party of armed Calvinist peasants, who attacked and murdered the friars. Fleming's body was carried to the monastery of Voticium, four miles away, and there buried.
Fleming was specially devoted to ecclesiastical history, his tastes in this direction being still further developed by his friendship for his learned countryman Father Hugh Ward. The latter, desirous of writing on early Christian Ireland, asked for his assistance. Even before Fleming left Leuven for Prague he had massed considerable materials and had written a Life of St. Columba. It was not, however, published in his lifetime. That and other manuscripts fell into the hands of Thomas O'Sheerin, a lecturer in theology at the College of St. Anthony of Padua who edited and published them at Louvain in 1667.
Fleming also wrote a life of Hugh MacCaghwell, Primate of Armagh, a chronicle of St. Peter's Monastery at Ratisbon (an ancient Irish foundation), and letters to Hugh Ratison on the lives and works of the Irish saints. The letters were published in The Irish Ecclesiastical Record.