Born into a family who were Plymouth Brethren, Underwood had his first paranormal experience at the age of nine, when he claimed to have seen an apparition of his father, who had died earlier the same day, standing at the bottom of his bed. During his childhood, his maternal grandparents lived for a time at Rosehall, a seventeenth century Hertfordshire house which it was claimed was haunted, supposedly having a bedroom in which guests claimed to have seen the figure of a headless man. Underwood's interest in hauntings and psychic matters began to take root at that time.
At the beginning of the Second World War, Underwood joined the publishing firm of J.M. Dent & Sons in Letchworth Garden City. In January 1942 he was called up for active service with the Suffolk Regiment. However, after collapsing at a rifle range at Bury St Edmunds a serious chest ailment was diagnosed which rendered him unfit for active service. Underwood was discharged from the army and returned to Dents. On 15 July 1944 Underwood married his wife Joyce at St. Mary's Church in nearby Baldock. She died in 2003 after having suffered with Parkinson's Disease for 14 years.
One of his early investigations was the Borley Rectory haunting, where, over a period of years, Underwood traced and personally interviewed almost every living person who had been connected with what the press had dubbed the 'most haunted house in England'. He built up a volume of correspondence with paranormal investigator Harry Price and after Price's death, he became literary executor of the Harry Price Estate. Underwood is a long-standing member of the Society for Psychical Research.
For some years Underwood was the Honorary Librarian of the Constitutional Club and the Savage Club, where he is a former Member of the Qualifications Committee. In 1976 a bust of Underwood was sculpted by Patricia Finch, winner of the Gold Medal for Sculpture in Venice.
Having been invited to join the Ghost Club by Harry Price, Underwood was its President from 1962 to 1993, when he left the group to found the Ghost Club Society of which he is the Life President. In recognition of his more than seventy years of paranormal investigations, Underwood accepted the invitation to be the Patron of The Ghost Research Foundation (founded in Oxford in 1992), who term him the King of Ghost Hunters, and he has recently accepted an invitation to be the Patron of Paranormal Site Investigators (UK).
At present Underwood is co-authoring two new books, and has written a Foreword to Damien O'Dell's "Ghostly Hertfordshire" - his home county.
Underwood is also the author of many books, mainly relating to the paranormal.
