He was born and educated in Geneva. He became tutor in the family of the count of Calenberg in Lower Saxony. In 1752 he was appointed professor of belles lettres to the academy at Copenhagen. He was naturally attracted to the study of the ancient literature and history of Denmark, his adopted country, and in 1755 he published the first fruits of his researches, under the title Introduction a L'histoire du Danemarch ou I'on traité de la religion, des maeurs, des lois, el des usages des anciens Danois.
A second part, more particularly relating to Danish literature, Monuments de la mythologie et de la poesie des Celtes, et particulierement des anciens Scandinaves, was issued in 1756, and was also translated into Danish. A translation into English, with notes and preface, by Bishop Thomas Percy, was issued in 1770 under the title of Northern Antiquities (republished with additions in 1847).
The book had a wide circulation, and attracted much attention on account of its being the first (though a very defective) translation into French of the Edda. The king of Denmark showed his appreciation by choosing Mallet to be preceptor of the crown prince. In 1760 he returned to Geneva, and became professor of history in his native city. While there he was requested by the czarina to undertake the education of the heir-apparent of Russia (afterwards the Czar Paul I), but declined. A more appealing invitation led to his accompanying Lord Mountstuart in his travels through Italy and thence to England, where Mallet was presented at court and commissioned to write the history of the house of Brunswick. He had previously received similar commission from the landgrave of Hesse-Cassel for the reparation of a history of the house of Hesse, and both works were completed in 1785.
The quietness of a literary life was rudely broken by the French Revolution, to which he was openly hostile. His leanings to the unpopular side were so obnoxious to his fellow-citizens that he was obliged to quit his native country in 1792, and remained in exile till 1801. He died at Geneva.
A memoir of his life and writings, by Sismondi, was published at Geneva in 1807. Besides the Introduction to the History of Denmark, his principal works are: