In 1797, Bonaparte chose him to accompany Marmont on his embassy to Rome to meet pope Pius VI, thinking that Prosper would make a good impression on the Romans as to the manners of the French army.

René Bouscayrol wrote of him as "a handsome, swarthy infantry captain" On 3 May 1798 Bonaparte left Paris to embark at Toulon, accompanied by Josephine and Jullien.

In Alexandria, Kléber wrote to Bonaparte on 22 August 1798, saying "I learned with true sorrow of the death of poor Julien [sic], your aide de camp".

Bourienne wrote about the investigation into the killing, saying "No one has found any trace of this sad event besides a jacket button in the dust of a hut, situated not far from Alkam.

We found this village deserted - all the inhabitants, of whom those most to blame had been captured, had got to know the time of our arrival and fled; we were thus forced to content ourselves with destroying this place by fire.

I do not guarantee that this is the burial place of chef de brigade Pinon, although I have been assured it is: for this officer took part in an expedition against the inhabitants of Darfur, in Upper Egypt; and I have Siout's notes on the trip.

His brother, general and Comte d’Empire, commissioned five plaster copies, of which two were placed in Jullien's houses at Lapalud, two at Vannes (including one at the prefecture).