Jayda Fransen, 33, was found guilty over a speech she delivered at a protest in August 2017.
She was also convicted for different comments at a peace wall in the Northern Irish capital.
The group’s leader, Paul Golding, 37, and two other men, Paul Rimmer and John Banks were acquitted on similar charges.
When convicting Fransen, the judge described her words as “a general, vehement attack against a religious group”.
She was told to return to Belfast Magistrates’ Court for sentencing next month.
All four defendants were on trial over speeches delivered at the ‘Northern Ireland Against Terrorism’ event in 2017.
They were accused of using abusive, insulting or threatening words intended to stir up hatred or arouse fear.
The court heard that Fransen told the attendees at the demonstrations that there was no moderate version of Islam and that “These people are baying for our blood.”
She went on to say: “Islam says every single one of you wonderful people here today deserves to be killed.”
The attendees of the rally were also told it was time for the world to come together against “the one common enemy”.
The judge told the court: “I’m satisfied these words were intended to stir up hatred and arouse fear.”
He also found her guilty over an unrelated incident which was filmed at a Belfast peace wall in December 2017.