Joseph was born in 1970 in Salisbury, Wiltshire to Mark (photgrapher and farmer) and Jini (novelist and artist, also known as Jennifer Lash). He has two older brothers (Ralph
, b. 1962, Magnus, b. 1965) and two older sisters (Martha, b. 1964, Sophia, b. 1967) and a twin brother, Jacob (a conservationist). His mother passed away in 1993, of breast cancer; she was 55. He has a foster brother, Michael Emery, who joined the family when he was eleven.
While Joseph was still a child, the Fiennes family moved 14 times between London, Wiltshire and Ireland.
He attended art school at 16, and spent two years at the Young Vic Youth Theatre in London. He also attended Guildhall School of Music and Drama. He is often referred to as Joe.
As of 2008, Joe is living with model, and former Miss. Switzerland, Maria Dolores Dieguez in his Notting Hill home.