Married Christopher Grierson blew more than £600,000 to install his lover in a New York apartment and shower her with cash and gifts during an 18-month affair.
The liaison ended after his Brazilian second wife found out but Grierson was left mired in debt and resorted to stealing from his employers, international law firm Hogan Lovells, by submitting false travel expense claims.
Grierson was jailed for three years today after admitting four counts of fraud at London's Southwark Crown Court.
The 60-year-old solicitor was paid £830,000 a year and involved in a string of major cases before his career ended in ignominy.
He once represented the Sultan of Brunei's brother Prince Jefri and also helped unravel the affairs of the collapsed Bank of Credit and Commerce International.
In another case, Grierson was instructed by liquidators in the US representing the estate of multi-billion dollar Ponzi scheme fraudster Bernie Madoff.
But his own life began to spiral out of control, and he suffered from depression and later heart problems. By 2010 he was working up to 3,500 hours a year - the equivalent of nearly 75 hours a week - and the following year he underwent a triple bypass operation.
His barrister Mark Ellison QC said that in 2005 he resorted to using call girls "as a means of escape and a source of some sort of affection".
Mr Ellison added: "In January 2006, he entered a totally different league. He became infatuated with a Lebanese woman in New York. He spent £630,000 paying rent on an apartment and paying over £280,000 to her in cash.
"This behaviour was truly bizarre and an indicator of the extent to which he had become derailed.
"This woman gave an impression of admiring him a little and of lifting his spirits.
"He must have lost all reason because he knew deep down that everything about this relationship was false.
"He had been a victim of manipulation, indeed, he might have been a willing victim."
Grierson borrowed £650,000 in a series of bank loans, largely to fund the affair, which ended when his wife found out about the relationship in August 2007.
But he was still saddled with debts, doubling his £900,000 mortgage to £1.8 million, exhausting his overdraft and maximising his credit cards.
Mr Ellison said: "He amassed regular monthly outgoings of £40,000 and just could not bear or dare to tell his wife what was going on."
In January 2008, he began stealing from his employers in order to "bridge the gap" so he could "provide the appearance of normality at home".
As a trusted senior member of the firm he was allowed to authorise his own expenses and although he had to account for them with receipts, he made "significant claims which were totally bogus", said prosecutor David Levy.
He was not caught until 2011 when a fellow partner at Hogan Lovells queried a £42,000 claim for travel between Heathrow to Los Angeles, which turned out to have been made against a dormant client account.
As the extent of his fraud began to unravel, Grierson realised "the game was up" and he announced his retirement. His colleagues realised that he had "for a long time abused his trust in a very, very cynical way".
He has now divorced from his wife and given her his £2.95 million home in Roehampton, south west London.
Grierson was able to borrow £1.3 million from a friend in 2011 to pay back the money he stole and since sold a property in France to repay that loan.
Jailing him, Judge John Price said he had carried out a "well-planned and sophisticated" crime.
He said: "You created false documents and you obtained substantial sums of money. You spent money to buy sexual favours and/or affection."