He was a feared serial killer who brutally severed the genitals of his male victims.

But after five decades behind bars, William 'The Mutilator' MacDonald was a scared little old man who was too terrified to be released from Sydney's Long Bay Jail.

MacDonald died of natural causes under police guard at Sydney's Prince of Wales Hospital on Tuesday morning aged 90.

He was jailed for life in 1963 for the grisly murder of four men in Sydney and one in Brisbane.

The convicted murderer became eligible for release 25 years after he was incarcerated, but despite encouragement for him to leave jail, MacDonald could not face life on the outside.

He told prison officers he was fearful of the world at large and that decades of confinement to his cell made him petrified of the world he once terrorised.

Australia's longest serving prisoner William 'The Mutilator' MacDonald has died aged 90 after spending 50 years at Sydney's Long Bay Jail

He took to the streets in 1961 and 1962 with a long-bladed knife and lured his victims, who were mostly down-and-outs sleeping rough around Sydney's inner city suburbs, into drinking sessions in parks and dark spots.

There he produced his knife and stabbed them before cutting off their penis and testicles.

MacDonald would later tell police this dark side of his personality was driven by a sexual assault upon him by an army corporal when he was a teenager.

But he appeared unable to control his compulsion and operated a brief but brutal reign of terror over inner Sydney from June 1961 to November 1962, after which he was charged with the murders of four men.

MacDonald died of natural causes on Tuesday morning aged 90 - fifty years after he committed his grisly crimes and was arrested on May 13, 1963

MacDonald was born Allan Ginsberg in Liverpool, UK, in 1924 and as a teenager enlisted in the Army, serving with the Lancashire Fusiliers.

He claimed he had been raped in an air raid shelter by a corporal while serving and, after being discharged in 1947, was diagnosed with schizophrenia and spent time in an asylum.

He emigrated first to Canada in 1949 and then to Australia, when he was 30 years old, in 1955 when he changed his name to William MacDonald.

He found a job as a railway worker and soon afterwards was arrested for obscenity after touching an undercover police officer in a public toilet.

As he later revealed, MacDonald's first murder victim was Brisbane man Amos Hurst, a man he met outside Roma Street station in Brisbane city after the hotels had shut on Saturday, March 19, 1960.

The pair went to 55-year-old Hurst's flat at Milton to drink heavily whereupon MacDonald was overcome suddenly with an 'impulse to kill' and put his hands around Hurst's throat 'and squeezed with all my might'.

When Hurst 'tried to struggle I hit him in the mouth' and then he 'put Hurst in the middle of the bed, took off trousers, shirt and shoes and put a sheet right over his head and tucked it in all round'.

The decomposition of Hurst's body when it was discovered and his history of heart disease initially led to a finding that he had died from natural causes, but by this time MacDonald had fled to Sydney where he embarked on a killing spree.

On June 4, 1961, a naked male body was found at the Sydney Domain Baths. The victim had been stabbed more than 30 times and castrated.

MacDonald, who would eventually confess to killing the man, Alfred Reginald Greenfield, and tossing his genitalia into Sydney Harbour, said he was compelled to murder Greenfield because he reminded MacDonald of the corporal who had raped him.

He had met Greenfield in Greens Park, Darlinghurst, near St Vincent’s Hospital, and lured him to the Domain baths to have a drink.

MacDonald then stabbed and killed Greenfield as he lay sleeping, again mutilating his genitals.

Five months later, a cook named Percy Nicholson found the mutilated body of Ernest William Cobbin at 5.25am on November 21, 1961, in Moore Park.

THE DAY I MET AUSTRALIA'S FIRST SERIAL KILLER It was 2005 and William 'The Mutilator' MacDonald had been locked up for more than 40 years. He was still living in the once feared Long Bay Jail, although by then, it was no longer the prison which housed the nation's most feared criminals. And anyway, MacDonald was by then as meek and mild as an inmate could be. He was in his early 80s and he looked as if he wouldn't hurt a fly. I was walking through Long Bay's aged and frail unit and we came upon Arthur 'Neddy' Smith getting around on his walking frame. The one-time big man of Sydney's crime scene who was doing life for murder, but had gradually succumbed to Parkinson's disease. Then we walked up a row of cells and came to one with the door open. Inside sat a little old man in prison greens, his hair white and his eyes big and watchful. My companion, the late Bob Stapleton, who had worked as a media representative for Prisons NSW for decades, whispered in my ear: 'That's old Bill MacDonald, the one who used to mutilate his victims'. Bob nodded at MacDonald who nodded back. I studied The Mutilator for a moment. There was nothing frightening or extraordinary about him, just an old bloke who looked meekly back. He didn't say a word. Later I heard a story from one of the prison officers that MacDonald had been considered for release, but the old man confided in the guards that he was too frightened to leave prison. The outside world he had terrorised four decades earlier now terrified him. Candace Sutton Advertisement

Detectives lead William MacDonald in handcuffs at the Criminal Investigation Branch after his arrest in 1963

MacDonald has spent 50 years in Sydney's Long Bay Jail (pictured) after he was arrested over the four murders back in May 1963

The discovery of MacDonald's next victim, who had been stabbed repeatedly and his genitalia severed in a public toilet, sparked public alarm and the then mystery killer was dubbed in the press 'The Mutilator'.

MacDonald had placed the genitalia into a plastic bag along with his knife and departed the scene.

He terrorised Sydney between 1961 to 1962, operating in The Domain, Moore Park, Darlinghurst and Concord areas. And he once threw a bag containing parts of his victim off Sydney Harbour Bridge.

A journalist who covered the case told the Sydney Morning Herald: 'It's difficult now to imagine the level of panic in the city... Homeless men were fighting to get into hostels."

And on March 31, 1962, he killed Frank Gladstone McLean, stabbing him six times in the neck in a Darlinghurst lane after a drinking session.

MacDonald sliced off McLean's genitals and placed them in a plastic bag, which he later discarded.

On November 3, 1962, MacDonald befriended a 42-year-old down-and-out named Patrick Joseph Hackett, whom he knew as McNulty, at the People's Palace in Sydney's city centre and they continued their drinking session at Concord.

MacDonald (top in his mugshot and police drawing) stabbed four men before cutting off their penis and testicles with a long-bladed knife on the streets of Sydney from 1961 and 1962

MacDonald claimed Hackett had reminded him of the corporal who raped him, and in a rage stabbed him in the neck while he was sleeping, then after a struggle stabbed him in the heart.

MacDonald hid Hackett's body under the linoleum of the kitchen in the Concord dwelling where it went unnoticed for three weeks.

After this murder MacDonald fled Sydney, for Brisbane, New Zealand and Melbourne. He used the alias Alan Edward Brennan, but was uncovered when a suspicious rail worker reported him to the police.

MacDonald admitted to the killings, saying his 'irresistible urge to kill' stemmed from his rape as a teenager, and was imprisoned at Long Bay Hospital.

In September, 1963, Justice McClemens sentenced MacDonald - then 39 years old - to jail for life.