Born in Pollokshaws, a suburb of Glasgow, in 1735, James Tassie was originally trained as a stonemason, then studied sculpture and modelling at the Academy of Fine Arts which had been established within the University of Glasgow by the printers Robert and Andrew Foulis. In 1763 Tassie moved to Dublin where he worked with Dr Henry Quin, ëKingís Professor of Physicí; together they invented a white vitreous paste (essentially a lead potash glass), which Tassie used to make the medallions which later made him famous.

In 1766 Tassie moved to London and quickly became known for making replicas of classical gems. He supplied casts to Wedgwood and made the first plaster casts of the celebrated Portland Vase, later reproduced by Wedgewood. The Empress Catherine the Great of Russia, an ardent admirer of his work, ordered a complete collection of his Pastes in imitation of gems and cameos.

The portrait medallions of eminent contemporaries, modelled from life, for which James Tassie is best known, were usually oval, either in the 'antique manner' with the subject wearing classical drapery, or in contemporary costume, showing the lace of cravats and style of wigs in great detail.

Tassie produced medallions of many distinguished men and women from the fields of politics, medicine, commerce, the arts, and the armed services. Many were Scottish, including professors at the Scottish universities.

James Tassie died in 1799 and was buried at Southwark, after which his business was carried on by his nephew and heir, William Tassie, who added many further items, including casts of coins, to an already extensive catalogue.