He initially worked for the French publishers Claude Duchet (Claudio Duchetti), Lorenzo Vaccari and Marcello Clodio.

[2] Thomassin was arrested in 1590 by the Holy Office on account of a portrait of the Protestant French king Henry IV of France which he had engraved.

The decision to produce prints after these earlier Tuscan and Northern-Italian artists was motivated by Thomassin's involvement with the Sienese art collector and dealer, Giulio Mancini (1559–1630).

[9] In the following six years, Thomassin continued to issue prints after drawings and paintings in Mancini's collection, including Parmigianino's The Virgin and Child in 1611, Vasari's Immaculate Conception in 1611–12, Zuccari's The Adoration of the Magi in 1613 and Francesco Salviati's The Baptism of Christ in 1615.

[11] Between 1608 and 1611/1612 Jacques Callot was present in Thomassin's workshop, where he was tasked with engraving prints after well known paintings: a commercial practice which offered a steady source of income.