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Mauha Huataki Fawcett, known within gang circles as 'Muck Dog', denied murdering Miss Manning, 27, on December 18, 2008.
But the Crown said the 26-year-old either took part in her brutal slaying at a gang pad that night or was party to it.
A jury of six men and six women took just under six hours over two days to return its unanimous verdict at the High Court in Christchurch today.
There were gasps of "yes" from a full public gallery as the jury's verdict was announced.
Justice David Gendall thanked the jury for their work.
Fawcett will be sentenced on May 1.
The prostitute was picked up by gangsters at her usual spot at Christchurch's red light district in what the Crown alleged had been a pre-planned and well-organised hit by the Aotearoa chapter of the Mongrel Mob where Fawcett would earn his gang patch.
She was driven the short distance to the Mob's pad at Galbraith Ave.
They took her inside a shed, turned up loud music, before raping her and beating and stabbing her with various weapons.
Once she was dead, they stood around her mutilated body and shouted 'Sieg heil' and barked like dogs, the four-week trial heard. The Crown contended that mobsters, including Fawcett, dumped her naked body in the Avon River just a few hundred metres away.
Miss Manning's partially naked body was discovered floating in the Avon River by a kayaker the day after she was killed.
It was the third prostitute murder in the city in as many years, but its brutality shocked senior, experienced investigators.
Fearing the gang would dob him in to police or else take him out over fears he would talk to police, Fawcett fled the city.
Over a series of police interviews, he incriminated himself in the killing, but later back-tracked. The Crown alleged he also made a confession to a former Mongrel Mob member, saying he had stabbed Miss Manning once.
During the trial he conducted his own defence, with assistance by an amicus curiae, and said he had lied throughout the interviews. He also claimed police had pressured and "coached" him into making false confessions.
Fawcett, who has a British bulldog tattooed on his face, said he hadn't been at the gang pad that night and denied having anything to do with her murder but the jury today rejected his version of events.