Len Castle trained as a secondary school teacher and started making pottery in 1947. Initially self-taught, he was the first potter to be awarded a fellowship from the Association of New Zealand Art Societies, travelling to St Ives to work with Bernard Leach in 1956-57. In 1963 he gave up teaching to become a full-time potter. He was a founding member of the New Zealand Society of Potters in 1963 and has been a regular exhibitor in the annual New Zealand Studio Potters exhibitions, which started in 1967.
In 1966-67 Castle studied in Japan and visited potters in Korea and China, returning there in 1974 as a member of a cultural delegation sponsored by the New Zealand Department of Foreign Affairs, when he also visited potters in Korea and China. He returned to influence a generation of potters. In 1991 he was one of twelve New Zealand ceramicists invited to represent New Zealand at the World Expo in Seville, Spain.
Castle received a CBE for Services to Pottery in 1986, a New Zealand Commemorative Medal in 1990. He received an Arts Foundation of New Zealand Icon Award in 2003, and a Distinguished Alumni Award from the University of Auckland in 2004. He was also awarded a DCNZM (Distinguished Companion of the New Zealand Order of Merit) for Services to Pottery. Mountain to the Sea, a touring exhibition of some sixty works, toured to selected galleries around New Zealand throughout 2009 and into 2010.