He became one of the catalysts of the conceptual art movement and established APPLE (1969-73) the second of New York’s seven not-for-profit spaces. He exhibited throughout the museum, gallery and alternative art scenes and in 1974 the Serpentine Gallery, London hosted a major survey exhibition which brought together his pop and conceptual works.

Since the early 1980s Apple has complemented his installation practice with text-based works, like the Art Transaction and From the Collection series’ that draw attention to the network of relations between artist, dealer and collector. He returned to New Zealand in 1990 and by 1998 was working with Hort+Research on the branding of a new apple cultivar. He became a registered trademark in 2007 formalising his art brand status and investigations into the legal concept of intellectual property.

Currently he is working on collaborative research projects, such as the analysis of his gut microbiome with molecular biologist, Justin O’Sullivan at the Liggins Institute. In 2019 GNS scientist, Cornel de Ronde installed Apple’s large survey-pin in the Tararua Ranges to mark the Centre of the Extended Continental Shelf of New Zealand (2012-19). In November 2019 Apple returned from Vue International Artist Residency in Shanghai having completed a public art work about Rewi Alley, one of the instigators of the Gung Ho movement. Despite Covid-19 restrictions, he has opened an exhibition A Brand Looking for a Product (1962–2020) at MTG Hawke's Bay Tai Ahuriri which traces his name change up to the Billy Apple® Cider project. The long-awaited monograph, Billy Apple®: Life/Work by Christina Barton, art historian and Adam Art Gallery director has just been published.

Apple’s work is held in public collections such as the Tate Britain,
Scottish National Galleries of Modern Art, Philadelphia Museum of Art, Chrysler Museum of Art, Guggenheim Museum and Te Papa Tongarewa Museum of New Zealand. He has had over 250 solo exhibitions and been curated into more than 250 mostly institutional group exhibitions. Apple’s works (including his fundraising art projects) have raised more than a million dollars donated to charities and institutions throughout New Zealand. At the age of 84 he continues to act as an ambassador for the visual arts in New Zealand and internationally. He is represented by Starkwhite, Auckland; Hamish McKay, Wellington; Rossi & Rossi, Hong Kong and The Mayor Gallery, London.