Joly, 24 and 122kg, represented France at Under 20 level in the Six Nations and was selected for the 2007 IRB Under 19 World Championships in 2007 in Belfast.
He began his career with Massy before spending four seasons at Stade Francais Paris, where he gained Top 14 and Amlin Challenge Cup experience. He joined Bourgoin in 2011.
Montanella and Joly both made their debuts for London Welsh in the club's friendly against the Scarlets last Saturday.
"They're both very experienced in the French leagues, which is a tough environment for front five forwards," said London Welsh head coach Lyn Jones. "They're both good scrummagers and they carry well - they'll give us some much needed bulk."