Born in Auckland in 1924, Castle graduated with a Bachelor of Science in 1946 from University of Auckland and trained as a secondary school teacher, eventually taking a lecturing position at the Auckland College of Education. His first experience of pottery was as a 10-year-old, seeing Olive Jones demonstrating at the Auckland Easter Show. Castle began making his first pottery in 1947 and took night classes with Robert Nettleton Field at Avondale College, Auckland. In 1956 he moved to St Ives, Cornwall for a year to work with Bernard Leach and became a full-time potter in 1963. The same year he helped establish the New Zealand Society of Potters.
In the early 1960s he had an architecturally designed house built in the bush of the Waitakere Ranges at 20 Tawini Road, Titirangi, with a kiln and rail system out the back, and a low basement which allowed pottery to be exhibited. Turning room for tour buses was provided in the street outside. The Boyes family which bought the house demolished the kiln; however, the bricks from it form the paving round the lower part of the house, and shards from discarded pottery works can still be found amongst the clay soil of the bush behind.
In the early 1950s, Castle met Theo Schoon and Schoon decorated the surfaces of pots thrown and fired by Castle. In the 1960s Schoon introduced him to the geothermal areas of the central North Island of New Zealand. Castle has continued to photograph this landscape area, which is also reflected in his pottery.
Castle studied pottery in Japan, Korea and China in 1966–67. He named Shoji Hamada as one of his influences. He built a new house in South Titirangi with a larger kiln and even more extensive railway to serve it in 1972–73, which is still operating. In 1989, along with a number of other New Zealand ceramic and glass artists, he was commissioned to supply work for the exhibition Treasures of the Underworld for the New Zealand pavilion at the World Expo at Seville in 1991. This work is now in the collection of the Museum of New Zealand Te Papa Tongarewa.
Castle died on 29 September 2011.
In the 1986 Queen's Birthday Honours, he was made a Commander of the Order of the British Empire for services to pottery. Four years later, he was awarded the New Zealand 1990 Commemoration Medal. Castle was appointed as a Distinguished Companion of the New Zealand Order of Merit in the 2004 New Year Honours, for services to pottery. In 2009, he declined redesignation as a Knight Companion when the New Zealand government restored titular honours.
The book Len Castle: Potter, published by Ron Sang Publishing, won a 2003 Montana New Zealand Book Award for non-fiction.  Six years later, Lopdell House Gallery's Making the Molecules Dance won the Montana New Zealand Book Award for best illustrative non-fiction.
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- Glamorous New Zealand Homes in the Bush' The New Zealand Woman's Weekly June 4, 1962, cover, and p16-17
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- "Len Castle", Collections Online, Te Papa
- "Burials & cremation details". Purewa Cemetery and Crematorium. Archived from the original on 24 September 2015. Retrieved 9 May 2015.
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- "New Year honours list 2004". Department of the Prime Minister and Cabinet. 31 December 2003. Retrieved 26 December 2018.
- Biography Archived 3 April 2012 at the Wayback Machine, The Arts Council
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- James Mack, Making the Molecules Dance: Len Castle Ceramics: a retrospective exhibition 1947-1994 (Lower Hutt: The Dowse Art Museum, 1994)
- New Zealand Potters site
- Steve Rumsey, Len Castle, New Zealand Crafts 21, Winter 1987