Born in Iran, the 50-year-old was named in reports hours before being gunned down as the "cleric" who triggered a security lockdown in the heart of Australia's biggest city by unfurling an Islamic flag and holding several people captive in a chocolate cafe.
Arriving in Australia as a refugee in 1996, Monis was no stranger to controversy.
Convicted over sending offensive letters to the families of dead soldiers, he was on bail on charges of being an accessory to the murder of his ex-wife and for sexual and indecent assault, multiple Australian outlets reported.
His personal website, before it was taken down on Tuesday, featured a banner photo with graphic images of dead Arab children, with the caption: "This is an evidence for the terrorism of America and its allies including Australia. The result of their airstrikes."
A woman hostage stands by the front entrance of the cafe as she turns the lights off in the Sydney central business district. (AFP photo)
The gunman converted from Shiism to Sunni Islam, according to a post on his website. The Sunni-based Islamic State and other jihadist groups consider Shiites heretics, or rejectionists - "rafidi" - of true Islam.
"I used to be a rafidi, but not anymore. Now I am a Muslim, Alhamdu Lillah," he said.
On his website, self-styled Sheikh Haron, also known as Man Haron Monis, says his children have been "taken away by the Australian government" and that he is not allowed to contact them.
Monis was on bail for dozens of charges, including 22 of aggravated sexual assault and 14 of aggravated indecent assault, according to the Australian Associated Press.
Stemming from his time as a "spiritual healer", an additional 40 charges were levelled against him in October and he was also on bail for being an accessary to murder.
The Sydney Morning Herald said it was understood that days before Monday's siege, Monis had lost a bid to have the charges over the letters overturned.
Showing a flair for the dramatic, a photograph said to be from his Facebook page published by the Business Insider showed him in chains with a poster claiming "I have been tortured in prison for my political letters.
Monis has said the crimes he has been accused of in the past were attempts to smear him.
"These cases are in fact political cases against this Muslim activist, not real criminal cases."
Police push back a member of the public who tried to get into a building located near the Lindt cafe, where the hostages were being held. (Reuters photo)
He compared himself to Wikileaks' Julian Assange on his website, saying both were activists facing government-backed smear campaigns.
He also counted astrology, numerology, meditation and black magic among his expertise, according to reports.
Before the raid, Monis had reportedly demanded an Islamic State flag and a phone call with Australian Prime Minister Tony Abbott.
But police and Monis's former lawyer Manny Conditsis said the public could be assured that the siege was not the work of an organised terrorist group.
"This is a one-off random individual," he told the ABC. "It's not a concerted terrorism event or act. It's a damaged-goods individual who's done something outrageous."
The Australian newspaper called him a "self-styled sheikh" who had sent offensive letters to the families of dead Australian soldiers and was on bail on charges of being an accessory to the murder of his ex-wife.
It said the 50-year-old - shown in local media in a photo smiling and sporting a beard and a white turban - lived in Sydney's southwest and was "understood to be a fringe Islamist".