Lethbridge's heart condition worsened, to the extent that he was unable to attend his mother's funeral in 1970. He eventually required 24-hour care, and was transferred to Nuffield Hospital in Exeter, where he died in his sleep on 30 September 1971. His body was interred at the family plot in Heytesbury, Wiltshire. As the Branscombe house was owned by the Lethbridge Family Trust, Mina was obliged to move out after her husband's death, and she sold his belongings out of financial necessity. Mina also collected together Lethbridge's unfinished book with the help of writer Colin Wilson; together they assembled it into publishable form and it was brought out by Routledge in 1976 as "The Power of the Pendulum".