The Earl, whose previous marriages both ended in bitter divorce, proposed to Karen Gordon less than six months after announcing he had split up with his previous fiancée.
The couple will wed on June 18 at the Spencer family home, Althorp in Northamptonshire, where the Earl married both of his previous wives.
A spokesman for the couple said the ceremony would be a “private wedding” attended by family and close friends.
The 46-year-old Earl is understood to have started dating Mrs Gordon, 38, towards the end of last year after he and his previous girlfriend, Bianca, Lady Eliot, broke off their engagement.
The couple were photographed walking arm in arm in New York last November and had been spotted dining at the Wolseley restaurant in London’s Piccadilly in October, though Mrs Gordon’s identity was not known at the time.
Mrs Gordon, a mother of two and former model, was previously married to Mark Gordon, a millionaire film producer whose credits include Saving Private Ryan.
After the couple separated in 2003, she used her divorce settlement to set up her own charity, Whole Child International, which works to improve the care of orphaned, abandoned, abused and neglected children in developing countries and has been praised by the Dalai Lama.
The Earl’s decision to marry again may come as a surprise to some of his friends, given the unhappy circumstances in which his previous marriages ended.
In 1989, the Prince of Wales and the Earl’s sister, Diana, Princess of Wales were guests at his first marriage, to former model Victoria Lockwood, with whom he went on to have four children now aged between 16 and 20.
The Earl had proposed to Miss Lockwood just 10 days after they met but, in 1997, just weeks after the Princess of Wales’s death, the Earl was exposed as a serial adulterer in a divorce hearing in South Africa, where the family lived.
Miss Lockwood claimed the Earl had 12 girlfriends in five months, and one of his former lovers, Chantal Collopy, a model, offered to testify against him at the hearing after discovering he had been seeing her while his wife was being treated for anorexia and drug addiction.
The woman who accompanied the Earl to the Princess’s funeral was Josie Borain, a Calvin Klein model who later became a friend of Miss Lockwood.
In his eulogy he suggested that his sister’s “blood family” were the best people to look after her sons, Princes William and Harry.
He married his second wife, Caroline Freud, in 2001. The couple had two children, now aged seven and four, but divorced in 2009. Mrs Freud fought her husband for the family’s London home, where she had hoped to bring up the couple’s children, but the Earl won and his ex-wife was forced to move out.
The Earl had left his second wife when the couple’s youngest child was four months old, after beginning an affair with Coleen Sullivan, an American television reporter whom he met when she interviewed him. Miss Sullivan put her career on hold to begin a new life with the Earl, but their relationship was over after 18 months.
Last year the Earl became engaged to Lady Eliot but parted in September.