Charles Jencks, the architectural historian, co-founder of the Maggie’s Centres and designer of cosmic gardens, who has died aged 80, pinpointed the expiry of modernism to a precise moment in history. “Modern architecture died in St Louis, Missouri on July 15, 1972, at 3.32pm (or thereabouts),” he wrote, “when the infamous Pruitt-Igoe scheme, or rather, several of its slab blocks, were given the final coup de grace by dynamite.”

He was referring to the notorious failed housing estate , which he thought embodied everything that was wrong with the dogmatic principles of modern architecture and urban planning. In its place, he announced, would come the brave new epoch of “Post-Modern” architecture, a style of “radical eclecticism” that would return content, meaning and metaphor to the built environment. With his pithy, polemic writing, he saw to it over the coming decades that the style wars were never far from the headlines.

As the unrivalled godfather of postmodernism, Jencks wrote, spoke and enthused in his characteristically charming, witty and debonair manner, for 50 years about architecture that embraced pluralism and difference over standardised homogeneity.

He wrote more than 30 books, constantly writing and rewriting the history of 20th-century architecture as we know it. He was forever coining new “-isms” and categorising movements and sub-movements in his great architectural taxonomy – from “haute vulgarisation” to “cosmogenic” and “organi-tech” – applying labels to architects who did not always welcome them.

His hallmark was an ever-expanding “evolutionary tree”, a complex diagram that attempted to visualise the messy history of architectural movements as a tangled matrix of “tortuous blobs”, which looked like a cross-section through a wormery.

A striking figure, often dressed in a purple velvet suit, deep-brimmed hat and a scarf rakishly draped over one shoulder, Jencks embodied the flamboyant American academic. To some critics, his journalistic style and lack of rigid consistency put him more in the category of zeitgeist chaser than serious historian, but his ideas were all the more influential for it.

His key book, Modern Movements in Architecture, published in 1973 and largely based on his PhD thesis, argued that modernism was a much more diverse affair than previous histories had claimed.

“Architectural traditions are rich and complex in their profusion and any attempt to reduce them to some simplistic notion of ‘modern’ or ‘the true style’ would be myopic and destructive,” he wrote. Four years later, The Language of Post-Modern Architecture became an instant bestseller and staple of reading lists around the world, running to 11 editions, each version adding a new cast of characters to the PoMo pantheon.

From Robert Venturi, Bruce Goff and Charles Moore, to Aldo Rossi, James Stirling and Hans Hollein, Jencks’ was a broad and welcoming church. As time went on, he became wary of how the style was being co-opted by big business, railing against “commercialised cliche” as postmodernism fell out of favour. But he lived long enough to see the movement’s ideas revived by a younger generation, and his own work championed by a new audience.

Born in Baltimore, Maryland, Charles was the son of Gardner Jencks, a pianist and composer, and his wife, Ruth (nee DeWitt Pearl). He attended Brooks school in North Andover, Massachusetts, and received his bachelor’s degree in English literature at Harvard University (1961), followed by a master’s in architecture from the Harvard Graduate School of Design (1965), after which he moved to the UK. He studied for a PhD in architectural history at University College London under the radical modernist historian Reyner Banham, from whom he said he learned much, “especially how to enjoy disagreements”.

While most known for his writing, Jencks had a parallel career as a designer, the combining of multiple roles being itself part of his postmodern agenda. His first built structure was a small studio in the woods of his family home in Cape Cod, which he called the Garagia Rotunda, a cheap shed with stuck-on mouldings and playful details, which characterised his penchant for “ad hoc” bricolage.

His stucco-fronted house in Holland Park, London, became a riotous living monument to his theories, adapted over the years in collaboration with postmodern architects such as Terry Farrell and Michael Graves and the sculptors Celia Scott and Eduardo Paolozzi.

19 Lansdowne Walk, Holland Park, London – Charles Jencks’ Cosmic House. Photograph: Chris Redgrave/Historic England

It features pedimented bookshelves, a sundial window seat, and a jacuzzi in the form of an upside-down classical dome, every corner alive with symbolism and allusion. It was grade-I listed by Historic England last year and plans are under way to convert it to house an archive museum, the Cosmic House, which will be open to the public by appointment.

Jencks’ keen interest in the larger forces of cosmology found its greatest expression in his landscape projects, which began when he designed the garden of his family home in Dumfriesshire, Scotland, with his second wife, Maggie Keswick, an expert in Chinese gardens.

Beginning in the 1980s, the Garden of Cosmic Speculation was conceived as a microcosm of the universe, a dizzying place of spiralling mounds, zig-zagging staircases, and distorted chequerboard terraces that ramble their way across 30 acres. Jencks went on to develop his cosmic landform principles in projects in Edinburgh, Milan, New York, Cambridge and Suncheon, South Korea.

The Garden of Cosmic Speculation, Dumfries, Scotland by Charles Jencks and Maggie Keswick: the Snake Mound and the Snail Mound.

Photograph: Alamy

One of his biggest works was Northumberlandia, aka the Lady of the North, a huge effigy of a recumbent naked woman with 34-metre-high grassy breasts, next to an opencast mine north of Newcastle. His outré landscape designs were dismissed by some as cod-scientific kitsch, but, as Jencks was fond of saying of his own extraordinary home: “If you can’t take the kitsch, get out of the kitchen.”

Beyond the undulating mounds, Jencks’ most lasting physical legacy lies in the programme of building beautiful and uplifting cancer care centres that he initiated with Maggie, when she was diagnosed with terminal cancer in 1993. Now counting 24 locations across the UK and abroad, the Maggie’s Centres mark a stark contrast to most clinical hospital environments, providing warm, welcoming homes-from-home for people living with cancer to receive support, and not “lose the joy of living in the fear of dying”, as Maggie put it.

Drawing on Jencks’ extensive address book, the charity has seen centres designed pro bono by, among others, Frank Gehry, Zaha Hadid and Rem Koolhaas, each time rethinking what a medical environment might be.

In 1961, Jencks married Pamela Balding and they had two sons. They divorced in 1973, and his marriage to Maggie came five years later; they had two children, John and Lily, and she died in 1995. He is survived by his third wife, Louisa Lane Fox, whom he married in 2006, and his four children.

• Charles Alexander Jencks, landscape architect, designer and writer, born 21 June 1939; died 13 October 2019