Mann was born in Yorkshire. Little is known of his education except that he seems to have imbibed deistic ideas in his youth. He left England about 1754 and went to Paris.
Here the study of Bossuet"s Discours sur l"histoire universelle exerted a profound influence upon him, and in 1756 he was received into the Catholic Church by the Archbishop of Paris.
Upon the outbreak of the war between France and England in the same year, he went to Spain, where he enlisted in a regiment of dragoons, and afterwards became a student at the military academy of Barcelona. He became prior of his monastery in 1764, but left the order thirteen years later, after having obtained a Bulletin of secularization and also the privilege of possessing a benefice. He took up his residence at Brussels and received a prebend in the Chapter of Notre-Dame de Courtrai.
In 1787 he was chosen perpetual secretary of the Brussels Academy, and carried on numerous meteorological observations under its auspices.
The invasion of the French in 1794 forced him to leave Belgium, and, after travelling in Germany and England, he finally settled at Prague, where he continued his literary labours until his death in 1809. Mann was a laborious student and a versatile writer
He is said to have refused the Bishopric of Antwerp offered him by Emperor Joseph II, rather than abandon his favourite studies.
Royal Society.