Why is Monsieur pronounced differently?
The reason why the two pronunciations are different is due to a process of Anglicisation, where English speakers make a borrowed word conform to native phonology.
How do you pronounce Monsieur?
How do you pronounce Bonjour Monsieur?
Is N silent in Monsieur?
Words in French with an “on” sound are very nasal. You don’t really pronounce the “n” at all, and it’s more like “aw,” but with your lips pursed and your cheeks pulled right in. In “monsieur,” though, you don’t pronounce it like that, in fact you hardly pronounce the “on” at all.