Miller was born in 1946, and raised predominantly in London and Manchester. His mother, who encouraged the artistic vocation, was a theatrical millner for one of the leading costumiers to the film industry, which, with cinema, he cites as an early inspiration:

As a child Miller experimented with coloured pencils and poster paints producing images of Ancient Egyptians during something he refers to as his 'Ancient Egyptian Phase', followed later by an obsession with cowboys and Indians. At the age of nine Miller attended Mortbane Academy for Boys in Invernesshire, Scotland, where he recalls regular painting expeditions to the surrounding countryside under the tutelage of the art master, nicknamed 'Old Dribble'. Between 1963 and 1967 he enrolled at Northwich School of Art, before embarking upon a degree at St. Martin's School of Art (now Central St. Martins College of Art and Design), London where he began in sculpture before switching to painting, and graduating with honours in 1970. Shortly after this he was taken on by an agent and began working in London as a professional illustrator.

Miller's earliest work included magazine and book jacket illustrations, including a host of illustrations for paperback titles by H.P.Lovecraft, and work for Men Only and Club International.

In 1975 and 1976 whilst Miller was staying in San Francisco, he was approached by Ralph Bakshi and invited to contribute to the film Wizards. Miller relocated to Los Angeles and worked on the animated movie, later citing it as an experience that left a profound impression upon him. He would later go on to work on the film Coolworld by the same director in the 1980s, and produce pre-production work for the film Shrek in the 1990s.

Miller is well-known for his work for the Fighting Fantasy gamebooks which rose to popularity in the mid-1980s, providing covers for early titles in the series like The Citadel of Chaos, House of Hell and Creature of Havoc. He has also contributed to the Games Workshop-published fantasy gaming periodical White Dwarf in which he was featured in an Illuminations exposé in issue 86, and provided numerous illustrations for various role-playing and war gaming books and supplements published by Games Workshop during the latter half of the 1980s, including the covers for Terror of the Lichmaster, Death on the Reik and Warhammer City for Warhammer, and a host of illustrations for the Realm of Chaos supplement and the first edition ofWarhammer 40,000. In the following decades Miller went on to provide further illustrations for gaming lines published by other companies, including the Everway, Shadowrun, and Earthdawn RPGs.

Miller has illustrated cards for the The Gathering collectible card game.

Miller is also noted for his Tolkien-inspired illustrations, and contributed to the lavishly illustrated A Tolkien Bestiary and Characters from Tolkien - A Bestiary, and has provided illustrations for British science fiction periodical Interzone and cover and interior images for SF titles like Alien Stories 2 by Dennis Pepper.

A number of anthologies of Miller's work have been published over the years. His first, with James Slattery, The Green Dog Trumpet and Other Stories, was published by Dragon's Dream in 1979, and was followed by another, Secret Art, and a third, entitled Ratspike, co-authored with fellow illustrator and Games Workshop art director John Blanche and published by GW Books. Miller has also produced imagery for two graphic novels, the first, The Luck in the Head, with writer M. John Harrison and a second with James Herbert called The City, as well as working on an unpublished third called Suzie Pellet.

Miller has exhibited frequently during his career in both solo shows and group exhibitions in Britain and internationally.

Current projects include the production of a series of black and white panel drawings called Corpus Pandemonium, and a book called The Broken Novel, a reworked film project called The Confessions of Carrie Sphagnum, a set of Tarot cards, and a theatre project entitled The Shingle Dance.

Miller's style is variously described as surreal, gothic and nightmarish or grotesque. "Edgy and surreal, Miller combines intelligent geometric exactness with a messy, fluid sense of what it means to be human." As fellow contemporary illustrator Patrick Woodroffe comments in the introduction to Blanche and Miller's Ratspike:

According to Miller, his illustrations have a tendency to the 'frontalistic', and are also noted to often feature recurrent elements inspired by fishes, flies and robotic forms, and the gnarled haunting trees which he claims originated in an attempt to cover failures of draughtsmanship. Says Miller of his work:

Miller cites amongst his principle artistic influences Albrecht Dürer and Leonardo da Vinci, the German Expressionists and French Impressionists, as well as an early predilection for Japanese landscape artists. Other sources include the formative influence of writer Alfred Bester, and a love for the Flash Gordon RKO Radio serials, and his early exposure to the cinematic medium in general which he feels lent a narrative quality to his work, as well as, of course, the every day world itself:

Miller has experimented with various media during his career, but has a preference for pencils, technical pens, watercolour, and charcoal. "I found self-expression with the pen - with oils it was quite the opposite." He also occasionally combines collage and photography into his pieces. His best-known published work has tended to be characterised by a trademark pen-and-ink and wash technique executed on line board and which he refers to as his 'Tight Pen Style', emphasising line detail and a restricted use of colour, something he views as a result of both short-sightedness and Northern European proclivities:

More recently he has taken to adding an Apple Macintosh computer to his range of tools.