Over the years, there have been unconfirmed sightings of him from South America to Africa but his whereabouts and his fate remain a mystery.
Lady Lucan has been interviewed for the ITV documentary: My Husband, The Truth, which includes unseen family footage of the Lucan family.
She reveals that relations between her and her husband became strained almost as soon as they settled down to married life.
She said: "He talked to me more before our marriage than ever he did afterwards. He said, 'That's the point of being married, you don't have to talk to the person.'"
In the interview, Lady Lucan talks about her husband's reckless lifestyle, his heavy debts and the bitter custody battle over their three children, which the Earl lost just days before Miss Rivett's murder.
She will also offer up her own theory of what happened to her husband. In 2013 speaking to The Mail on Sunday she said: "I believe my late husband committed suicide shortly after the murder of Sandra, most probably by bravely throwing himself on to the propellers of a ship in mid-Channel, hoping that his remains would be irrecoverable so that death duties would not be immediately payable as the children's education had not been secured.
"My husband committed suicide because he was an honourable man." She added: "Of course, I have learned to forgive him." At the time, she was commenting on the events of 1974 being made into a drama which starred Rory Kinnear as her husband.
She said: "I don't think it is right that a criminal matter should be used for entertainment. The film is based on the most absurd book I have read on the Lucan affair so I didn't think it was sensible to have anything to do with it."
According to reports, she has been paid £65,000 to take part in the new documentary.
She is believed to have a strained relationship with her three children. In 1999, Lord Lucan was pronounced dead and last year a High Court judge issued a death certificate meaning his only son George was free to inherit the earldom.
Even more than 40 years on, the mystery of Lucan's disappearance is a mystery of abiding fascination for the public.
Among the numerous theories put forward are that he was kidnapped by the IRA, or that he shot himself dead and was then fed to tigers in a zoo in Kent owned by his friend, the late John Aspinall.
There have been claimed sightings of Lucan across the world, including France, Australia and Cape Town, where police examined a beer glass for his fingerprints.
Ten years ago, he was said to have been living in a car in New Zealand. In another version of events, he had renamed himself 'Jungly Barry' for a new life as a hippy in India.
Lady Lucan's daughter, Camilla Bingham, who is a QC, declined to comment on the documentary last night. The programme will be shown on Monday, June 5.
