As militants attempted to break government defences in strategic areas and edge closer to Baghdad, Christians joined hundreds of thousands of Shiite and other refugees into Kurdistan.
The (so-called) Islamic State in Iraq and Levant (ISIL) group running Mosul had already demanded that those Christians still in the city convert, pay a special tax or leave but messages blaring on mosques' loudspeakers appeared to spark an exodus.
An earlier statement by Mosul's new rulers had said there would be "nothing for them but the sword" if Christians did not abide by those conditions before noon (0900 GMT) on Saturday.
"Christian families are on their way to Dohuk and Arbil" in Kurdistan, Chaldean patriarch Louis Sako, who heads Iraq's largest Christian community, told AFP.
"For the first time in the history of Iraq, Mosul is now empty of Christians."