The 50-year-old, a self-styled Muslim cleric from Iran, was already known to Australian authorities as he had been charged with the indecent and sexual assault of a woman in western Sydney in 2002.
Monis, who has his own website in which he compares himself to WikiLeaks founder Julian Assange and declares his conversion from Shia to Sunni - was also charged with being an accessory to the murder of his ex-wife and mother of two. He is currently on bail.
In October, he was also charged with an additional offences, including 22 counts of aggravated sexual assault and 14 counts of aggravated indecent assault relating to six other women.
It is believed that Monis - whose previous name when he lived in Iran was Manteghi Bourjerdi - worked as a healer in Sydney and attacked his victims during healing sessions.
Monis also sent "grossly offensive" letters to the families of dead Australian soldiers. The letters were allegedly sent in retaliation to Australia's military involvement the battle against the Taliban in Afghanistan. He was sentenced to 300 hours of community service.
Monis rejected the charges and claimed that the letters were his version of "condolence cards" and that the charges were "politically motivated".
In a 2013 letter addressed to Australia's Prime Minister Tony Abbott, Monis said that Australia's policy in Afghanistan "has a significant role in jeopardising security and peace in the world especially in Australia."
Monis claimed he was a spiritual leader, or ayatollah, and an expert in astrology, numerology, meditation and black magic. His views, however, led to the abduction of his wife and daughters.
The cleric fled to Australia in 1996 and changed his name, also assuming the title of Sheikh Haron.