Mr Cheng's death shocked the nation and led police to arrest, charge and then jail four individuals behind the brazen and deadly extremist Muslim terror attack.
On Friday, a Parramatta Supreme Court jury found Mustafa Dirani guilty of one count of conspiring with others to do an act of terrorism.
A separate backup charge of supplying a firearm to an unauthorised person was not considered after the jury's verdict for the primary offence.
Mr Cheng was shot in the head by 15-year-old Farhad Mohammad outside the NSW Police Force headquarters in Parramatta at 4.30pm on October 2, 2015.
This is the fourth time Dirani has faced court over the charges. In 2018, the first jury was discharged after being unable to reach a verdict.
In 2019, he was found guilty and sentenced to a maximum of 28 years in jail. This was overturned on appeal, prompting a third trial in which the jury was again unable to reach a verdict and was discharged in 2022.
During the latest retrial which began in February this year, jurors heard Dirani made extremist posts, including about beheadings and suicide bombs, in the months before the attack.
Dirani and another man Raban Alou were accused of supplying a 1942 Smith & Wesson revolver to Farhad at the Parramatta Mosque.
The teen then walked to police headquarters dressed in traditional Islamic clothing, shot Mr Cheng and was himself killed after engaging in a gunfight with police security guards.
Dirani always denied he had any connection to the plot, claiming he had followed Alou around western Sydney in the hours before the attack because he was waiting to get something to eat with his friend.
Prior to the shooting, the pair met Talal Alameddine who supplied them the gun, with police surveillance capturing their movements at various parks, car parks and other locations.
While no footage of the actual gun changing hands was taken during these interactions, the jury found Dirani guilty of his involvement in the terrorist plot despite crown prosecutors' circumstantial case against him.
Alou was previously sentenced to a maximum of 44 years behind bars for his role in the plot and Alameddine jailed for up to 17 years and eight months.
A third man Milad Atai, who shopped around for the gun with Alou, was jailed for up to 38 years.
Dirani's matter will next come before the court on December 29.
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