Mr Young, 52, who was sent to prison during a vitriolic and very public divorce row over a multimillion-pound settlement, fell on to railings outside the luxury flat in Montagu Square, Marylebone, central London, in what is said to have been a “grisly” and “brutal” scene.
The tycoon’s American model girlfriend, Noelle Reno, said in a statement: “I am distraught by the sudden loss of my best friend and ask that you give me the respect and privacy I need to grieve in peace.”
Ms Reno, a 30-year-old fashion entrepreneur, acquired a level of fame after appearing alongside Mr Young on reality television show Ladies of London.
Police said the death, which happened on Monday evening, is not being treated as suspicious.
Mr Young’s ex-wife Michelle, 50, who was locked in a long legal battle with him, said: “I just want to be left alone.”
She told The Telegraph earlier that she was with her daughters and said they “have been to hell and back”.
A neighbour said Mr Young and Ms Reno had been living for a few months at the property, which is believed to be rented and is opposite the Swedish embassy and next to the embassy of Switzerland.
He said the couple had been in the flat together on the day the businessman died.
Another local man paid his respects by laying a white lily at the boarded-up railings. The man, who did not want to be named, said: “I live round the corner and saw the aftermath of what happened.
“It was a pretty grisly scene, so brutal.
“I saw it was Scot Young in the news and decided to bring a flower out of respect. It’s such a human tragedy.’’
A spokesman for Scotland Yard said police were called to Montagu Square at 17.18pm on Monday to reports of a man having “fallen from a fourth-floor window’’.
He added that a man, believed to be in his early 50s, was pronounced dead at the scene. “The death is not being treated as suspicious at this time,’’ he said.
According to The Telegraph, one associate who arrived to lay flowers at the scene, said: “We don’t know if it was suicide yet.”
In March, a High Court hearing was told that Mr Young’s estranged wife Michelle remained empty-handed more than three months after she was awarded £20 million by a judge.
In November last year, Mr Justice Moor assessed Mr Young to be worth £40 million - after analysing evidence at a trial - and said Mrs Young was entitled to half of that.
The judge said the businessman might have hidden assets offshore. Mrs Young said she thought he was worth “billions’’ and claimed he had hidden a “vast fortune’’.
Mr Young said he had suffered a financial “meltdown’’ seven years ago and was bankrupt and penniless, with debts adding up to £28 million.
The judge described the litigation - which began more than six years ago - as “quite extraordinary’’.
He said over the years Mrs Young had run up a legal bill of more than £6million and employed 13 sets of lawyers and four teams of accountants.
The Youngs, who have two daughters, separated in 2006 after starting a relationship in 1989.
Mr Justice Moor said, when ruling on the Youngs’ dispute in November 2013, that the couple suffered “serious ill-health’’ throughout the case.
He said: “It is a sad fact that both parties have suffered from serious ill-health during the course of these proceedings.
“I have made orders preventing the media from reporting any details in relation to the husband and I would have made a similar order in relation to the wife had I been asked. There is, however, no restriction against reporting what now follows.’’
He added: “I do not intend to deal further with health issues other than to say three things. First, there have been suggestions by the wife at times that the husband was feigning his ill-health. I reject that completely. I have seen the medical reports and I accept them.
“Second, I do accept that the strain of this litigation is likely to have been a contributing factor to the ill-health of both these parties. Third, I do not consider it necessary to go into further detail because it does not appear to me that their health is relevant to the exercise I have to perform.
“So far as the wife is concerned, I very much hope that her ill-health is completely behind her. So far as the husband is concerned, he has not said that he is unable to work going forward.
“Provided he takes his medication and absent the strain of these proceedings, I am satisfied that he will be able to control his health difficulties and rebuild his life.’’
Mr Young and Ms Reno recently appeared together in Ladies Of London, alongside model Caprice and other London socialites.
The programme is similar to the Real Housewives franchise and the final episode was aired earlier this month on ITVBe.
During the series, which also starred socialite and businesswoman Caroline Stanbury and the late Alexander McQueen’s muse Annabelle Neilson, Ms Reno referred to Mr Young’s divorce battle and the press coverage of it.
Cameras followed the couple as they viewed plush places to potentially rent in London as well as showing them in social situations with their co-stars.
Ms Reno’s most recent tweet, posted on December 7, said: “No matter what the day throws, I know I can handle it if my day starts with meditation (even if it’s day #4 of insane xmas parties!!)’’