Born in Callander, Stirlingshire, on November 25, 1897, Duncan made her name as a medium with seances in which she appeared to summon the spirits of recently deceased persons by emitting ectoplasm from her mouth. In 1931 Duncan was examined by the London Spiritual Alliance, who denounced her as a fraud. In 1934, during a seance in Edinburgh, a sitter grabbed one of her materialisations, which turned out to be a stockinette undervest. Duncan was found guilty of "affray" and fake mediumship at Edinburgh Sheriff Court, and was sentenced to a £10 fine or one month in prison.
At one of her seances, witnessed by a group of stage magicians led by William Goldston, founder of the British Magicians Club, the Great Lafayette is said to have materialised. The Great Lafayette (1871-1911) was a stage illusionist who had died during a performance at the Empire Theatre in Edinburgh, when the set for one of his illusions, ‘the Lion's Bride’, developed an electrical fault; Lafayette died, trying to rescue his stallion ‘Arizona’ from the flames while the orchestra played “God Save the King”. Ten of his players also died, including a 15-year old girl trapped in a mechanical teddy bear, and Lafayette's body double whose body was at first mistaken for that of Lafayette himself. Lafayette's body was discovered by workmen some days later, when the mistake was realised. Lafayette is buried alongside his terrier 'Beauty' who had died of overeating a few days before.
In 1941, during World War II, Duncan held a seance in Portsmouth at which she indicated knowledge that HMS Barham had been sunk. This was true, but had been kept secret to avoid its demoralising effect on public opinion. At the séance a spirit named Syd is alleged to have materialised; Syd apparently wore a cap with an HMS Barham band, and spoke with a relative. It seems however that although the sinking had not been made public, relatives of the dead – including many in Portsmouth – had been told by the Admiralty.
In January 1944, Duncan was arrested at another séance when a white-shrouded manifestation proved to be Duncan herself, in a white cloth. Duncan was tried under section 4 of the Witchcraft Act 1735, covering fraudulent "spiritual" activity, found guilty, and imprisoned for nine months.
Duncan's trial became a controversial topic. The Prime Minister, Winston Churchill, wrote a memo to the Home Secretary complaining about the misuse of court resources on the "obsolete tomfoolery" of the charge.
After her release in 1945, Duncan returned to conducting seances. In 1956, the Nottingham police raided a séance Helen Duncan was giving; a few days later, on December 6, she passed over to the other side.