He was born at Cliftbog, Aberdeenshire, the son of Thomas Dempster of Muresk, Auchterless and Killesmont, sheriff of Banff and Buchan. According to Dempster himself, he was the twenty-fourth of twenty-nine children. He went to school in Aberdeenshire, and at ten entered Pembroke Hall, Cambridge; after a short while he went to Paris, and, driven out by the plague, to Louvain; from there, by order of the pope, he was transferred with several other Scottish students to the papal seminary at Rome. Being forced by ill health to leave, he went to the English college at Douai, where he remained three years and took his M.A. degree. While at Douai he wrote a scurrilous attack on Queen Elizabeth I of England, which caused a riot among the English students.
He showed such ability that, when still in his teens, he became lecturer on the Humanities at the University of Tournai. After a short stay, he returned to Paris, to take his degree of doctor of canon law, and became regent of the college of Navarre. He soon left Paris for Toulouse, which in turn he was forced to leave by the hostility of the city authorities, aroused by his violent assertion of university rights. He was than elected professor of eloquence at the academy of Nimes. A murderous attack upon him by one of the defeated candidates and his supporters was followed by a libel case, which, though he ultimately won it, forced him to leave the town.
A short stay in Spain, as tutor to the son of Marshal de Saint Luc, was terminated by another quarrel; and Dempster returned to Scotland with the intention of claiming his father's estates. Finding his relatives unsympathetic, and falling into heated controversy with the Presbyterian clergy, he returned to Paris, where he remained for seven years, becoming professor in several colleges. In the end, his temporary connection with the college de Beauvais was ended by a fight, in which he defeated officers of the king's guard, forcing him once again to change his place of residence. The dedication of his edition of Rosinus' Antiquitatum romanarum corpus absolutissimum to King James I had won him an invitation to the English court; and in 1615 he went to London.
His reception was flattering; but his hopes of preferment were dashed by the opposition of the Anglican clergy to the promotion of a papist. He left for Rome, where, after a short imprisonment on suspicion of being a spy, he gained the favour of Pope Paul V, through whose influence with Cosimo II, grand duke of Tuscany, he was appointed to the professorship of the Pandects at Pisa. He had married while in London, but had reason to suspect his wife's adultery. Violent accusations followed, indignantly repudiated; a diplomatic correspondence ensued, and a demand was made, and supported by the grand duke, for an apology, which the professor refused to make, preferring to lose his chair. He set out once more for Scotland, but was intercepted by the Florentine cardinal Luigi Capponi, who persuaded him to remain at Bologna as professor of Humanity. This was the most distinguished post in the most famous of continental universities, and Dempster was at the height of his fame. Though his Roman Antiquities and Scotia ilustrior had been placed on the Index pending correction, Pope Urban VIII made him a knight and gave him a pension. He was not to enjoy his honours long. His wife eloped with a student, and Dempster, pursuing the fugitives in the heat of summer, caught a fever, and died at Bologna.
Dempster'st position in the history of scholarship is due mainly to the versatility which made him equally at home in philology, criticism, law, biography and history. The obvious defects of his work are due to his restlessness and impetuosity, and to a patriotic and personal vanity which led him into absurd exaggerations and an incurable habit of romancing. The best known of his works is the Historia ecclesiastica genus Scotorum (Bologna, 1627). In this he tries to prove that Bernard (Sapiens), Alcuin, Boniface and Joannes Scotus Erigena were all Scots, and even Boadicea becomes a Scottish author. This criticism is not applicable to his works on antiquarian subjects, and his edition of Benedetto Accolti's De bello a Christianis contra barbaros (1623) has great merits.
Some of his Latin verse is printed in the first volume (pp. 306354) of Delitiat poetarum Scotorum (Amsterdam, 1637).
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