Born at Chichester, Sussex on 27 February 1938 to Joan Mary Moore, an Irish Catholic, and Brigadier Henry Alleyne Lash, a British colonial officer, Jennifer Lash lived in India where her father was stationed until the age of 6. When her family returned to England, they settled down in Surrey. Raised a Roman Catholic, Lash attended boarding school at the Convent of the Sacred Heart in Tunbridge Wells, Kent, and continued on to Farnham Art School. When she was 16 years old, her studies were cut short by family problems. She discontinued her education and moved to London where she supported herself with odd jobs to support her artistic pursuits.

In 1962, she published her second book The Climate of Belief and married Mark Fiennes with whom she raised 7 children: actors Ralph Fiennes and Joseph Fiennes, film makers Martha Fiennes and Sophie Fiennes, composer Magnus Fiennes, Jacob Fiennes, a conservation manager, and a foster son, Michael Emery, an archaeologist. The family frequently relocated and lived in Suffolk, Wiltshire, Ireland and London. Lash went on to write four more novels over the next twenty years. They are: The Prism (1963), Get Down There And Die (1977), The Dust Collector (1979) and From May To October (1980).

Lash's haunting paintings were featured in several exhibitions in places such as The Penwith Galleries in St Ives, The Halesworth Gallery and Westleton Chapel Galleries in Suffolk.

In the late 1980s Lash was diagnosed with breast cancer. While in remission from the disease, she travelled to Lourdes and Saintes Maries de la Mer in France and to Spain's sacred Santiago de Compostella. During this time she wrote her only non-fiction book, On Pilgrimage. Jennifer Lash lost her fight to cancer on 28 December 1993 at Odstock, Wiltshire, aged 55. Her final novel Blood Ties was published posthumously and is widely regarded as her finest work.