He became a lecturer in the short-lived revolutionary École normale in the year III (1794), and was employed by the Republican ministry of treasure from 1795 to 1802, a post which he left in order to study oriental languages.
He went to Cairo as translator in 1806, where he served as vice-consul (but was never promoted to consul because he refused to leave the mother of his children, a Ragusian laundrywoman, with whom he lived in concubinage).
He moved to Alexandria in 1816 and ended his career in the position of first dragoman (official interpreter) of the French consulate for Egypt.
An Ethiopian cleric known as Abu Rumi was travelling through Cairo and became very ill. Asselin took him in, provided for him, and engaged him to translate the Bible into Amharic.
But more significantly, he also provided Abu Rumi with writing materials and aided him with his knowledge of the original Biblical languages, Greek and Hebrew.