Sinden was born at St Budeaux in Plymouth, Devon. He grew up in Ditchling, Sussex. He was married to actress Diana Mahony from 3 May 1948 until her death from cancer on 15 September 2004, aged 77. They had two sons, actor Jeremy Sinden (1950–1996) and producer Marc Sinden (born 1954). Jeremy died from lung cancer, aged 45.