Here is what we know so far about the self-proclaimed "sheikh" and his previous run-ins with the law.
He was well known for sending 'hate' letters to families of deceased Australian soldiers
Monis was dubbed the "hate sheikh" after he sparked national outrage by sending a series of offensive letters to the families of Australian soldiers killed in Afghanistan from 2007-2009. Monis likened some of the soldiers to murderers and wrote in other letters that the soldiers were going to hell. His lawyer at the time said that Monis was "preaching peace."
After losing an appeal in which he argued that the letters were protected by the right to free speech, Monis pleaded guilty last year and was sentenced to 300 hours of community service.
He had a long rap sheet, including sexual assault and accessory to murder
This year, Monis was charged with being an accessory to murder in the death of his ex-wife, Noleen Hayson Pal, who was found stabbed and lit on fire in the stairwell of a Sydney apartment building last year.
Monis is also currently facing more than 40 sexual assault charges. The Sydney Morning Herald reported that Monis was arrested in April for assaulting a woman who sought his services as a "spiritual healer."
Monis allegedly told the woman that he was an expert in "astrology, numerology, meditation and black magic" and then assaulted her as part of a "spiritual healing technique."
He believed he was a political target and said he was tortured under police custody
Monis' personal website, which has since been taken down, identified him as a Muslim cleric and activist who has "continuously been under attack & false accusation by the Australian government & media since he started his political letter campaign from 2007," according to CNN.
On his site, Monis also compared himself to WikiLeaks founder Julian Assange. Assange has been holed up in the Ecuadorian embassy in London since being charged with sex crimes in Sweden, which the WikiLeaks founder has argued were politically motivated.
After being charged in connection with his ex-wife's death, Monis stood outside the court in chains to protest. He was holding a sign that said he had been tortured in police custody. Manny Conditsis, the lawyer who represented Monis at the time, told Australia's ABC News that his former client was treated poorly in prison.
"He was put through let's say some very unpleasant events, involving matters of excrement over himself and his cell," he said.
He was critical of Australian politicians and the country's policy on Afghanistan
Last week, Monis reportedly railed against "racist and terrorist" Australians who supported the United States' foreign policy while praising the terror group known as the Islamic State on his Facebook page, which has been taken down.
The Sydney Morning Herald reported that Monis' Facebook page, which had garnered more than 14,000 likes, stated that it was "'Team Islam' against Australian oppression and terrorism."
He also wrote a letter to Prime Minister Tony Abbott in 2013 challenging him to a debate over Australia's policy on Afghanistan, arguing that the country's mission there was "jeopardising security and peace in the world especially in Australia."
"If it is proven in our debate that the government's policy endangered Australia, if it is proven the government made Australia unsafe, if it is proven that Australia and Australians will be attacked, in that case I expect you to change the Australia's (sic) policy," he wrote in his letter to Abbott, as quoted by the Sydney Morning Herald.
He appears to have been acting alone during the siege
Monis made some of his hostages hold up a black flag in the window of the Lindt Chocolat Cafe, while he also reportedly requested that an Islamic State flag be brought to him. But Monis had no connections to IS or any other terror organization, according to his former lawyer.
Conditsis described his former client to Australia's ABC as an isolated individual who likely acted alone.
"His ideology is just so strong and so powerful that it clouds his vision for common sense and objectiveness," Conditsis said.