Phillips was born on 16 December 1932 in Uxbridge to Philip and Elsie Phillips (née Williams). He was educated at St Christopher School in Letchworth and after national service with the Royal Air Force in Canada at Chelsea School of Art. He next joined Ogilvy & Mather Advertising, where he rose to be art director. He turned freelance in 1968. He took pictures for the album Goodbye for rock band Cream, on a commission from his friend Alan Aldridge, and later took pictures for the Jack Bruce albums Songs for a Tailor and Out of the Storm.

He is best known for his photographic books of the British flora. The first, Wild Flowers of Britain, sold 400,000 copies in the first year. It was about the first plant identification book that had photographs rather than paintings of flowers. In total, his 20 books sold over 4.5 million copies worldwide.

He was also an expert on mushrooms and roses who wrote more than forty books on gardening and wild plants and fungi, many with Martyn Rix. He was also an Honorary Garden Manager at Eccleston Square in London, where he lived, and served as chair of the Society for the Protection of London Squares.