Justice Cameron Mander remanded Marong in custody to be sentenced on April 20.
Duckmanton's family said the guilty verdict was the "right result" but provided little closure or solace for them.
Outside court, Detective Inspector Darryl Sweeney read a statement on behalf of police and the family.
"This has been a harrowing case for everyone involved and the right result has been achieved today," he said.
"The result won't provide closure or solace for Renee's family, but it is important for everyone that they have answers and that the offender has been held to account.
"Renee's family would like everyone to remember her as a much loved daughter, sister, cousin, granddaughter and aunty - she will always remain a much loved part of the family."
Duckmanton's family also thanked Victim Support, the Crown prosecution team, police, and members of the public who assisted with evidence in the case - notably those who were first on the scene and found Duckmanton.
Harrowing evidence
During the harrowing two-week trial, 33-year-old Marong admitted strangling Duckmanton in May 2016 after picking her up from the city's red light district before setting her body on fire.
But he denied that he meant to kill her, and had no murderous intent, claiming he was insane at the time, suffering mental impairment and delusional, psychotic thoughts which began after voluntarily stopping his insulin medication early in 2016.
The jury, however, sided with Crown and its "overwhelming" evidence, dismissing Marong's claims that he had been "disconnected from reality" at the time he killed Duckmanton, and that from May 6-17, he was poisoned by kidney failure that resulted in urine "travelling in my brain".
A psychiatrist and a psychologist, who both assessed Marong before he stood trial, concluded that he did not meet the criteria for a defence of insanity.
Forensic clinical psychologist Ghazi Metoui found that Marong reported a settled mental state over the period "leading up to, during and subsequent" to the killing.
Dr Erik Monasterio, a forensic psychiatrist, found Marong's claims of insanity to be "farcical" and "preposterous".
His DNA matched samples taken from Duckmanton, and was also directly linked to samples found where her body was dumped.
Marong, whose internet history in the days before the killing showed searches for necrophilia, denied having sex with her dead body.
His movements on May 14, 2016 were recorded on CCTV as well as cellphone and bank records.
During his evidence, Marong said that after he had sex with Duckmanton in the back seat of his car, she wanted to return to Manchester St.
He said "that voice, that screaming, that yelling was agitating me" and the dispute "triggered something very serious".
"The only method I could use to stop it was to compress her neck," he said.
