[1] Arnoldi went to secondary school in England[2] then returned to Montreal to learn about medicine and received his medical licence in 1795.

In 1814, he signed an address defending Jonathan Sewell and James Monk against the Legislative Assembly of Lower Canada.

[1] In 1823, George Ramsay, 9th Earl of Dalhousie, the governor of Quebec, decided that only doctors from Montreal General Hospital could be examiners, which excluded Arnoldi.

That year, he met with Montreal doctors to petition stricter measures taken to teach and practise medicine in the colony.

[1] A new law was passed in 1831 that transferred the responsibility of appointing examiners away from the governor and towards a vote by licensed physicians in each area.

[1] In 1836 Arnoldi, in his role as a magistrate, ordered a constable to arrest a black serving girl who had left her mistress's home.

Arnoldi and his son, François-Cornelius-Thomas, were accused of desecrating the corpse of Jean-Olivier Chénier, a member of the Patriotes who was excommunicated as a Catholic for using firearms in church grounds where he was ultimately killed by government forces.

During this time he regularly spoke in favour of increasing standards and the regulations for teaching and practising medicine in the province.

[1] In 1847, the College of Physicians and Surgeons of Lower Canada was established to regulate the medical profession, and Arnoldi was appointed as its first president.

[1] One of his daughters, Élisabeth, married Benjamin Holmes, a leading figure in the Montreal banking community and a member of the Legislative Assembly of the Province of Canada.