John Miller was a kind and generous patron to many good causes, frequently supporting museums, churches and schools by the donation of paintings that could be auctioned, raffled or sold on behalf of the relevant mission. Taking an essentially religious perspective on life, John played a significant pastoral role in Anglican activities in Cornwall in later years. 

Arriving in Cornwall in the 1950s with his long-time friend, colleague and partner Michael Truscott, John's architectural skills were immediately put to use in the re-design and adaptations made to the Newlyn Art Gallery. His style in painting changed dramatically over time, from early figurative landscapes of great beauty and detail to later abstract sea and landscapes of vivid blocks of colour depicting the horizon where the sea meets the sky. 

An extensive collection of his work was donated by his estate, after his death in 2002, to be housed in the Sunrise Centre at Royal Cornwall Hospital, Truro, 15 of which are illustrated in the Public Foundation Catalogue review of work by artists held in public collections (pp165-7). All of the illustrated works are untitled and represent his style in later life. 

John was a lay canon at Truro Cathedral, and the creator of the large mural painting there: Cornubia-Land of the Saints.

His work was extremely popular, and he had many imitators.  It was not unusual to find a John Miller seascape on the back wall of a soap show such as Coronation Street, or a china mug at a local coffee shop.