Miles was born and educated in South Wales. He gained a degree in Modern History from Oxford University and spent three years as a lecturer, before becoming a full-time writer. Miles's early work was as a scriptwriter for television and radio, including series such as Crossroads, Z-Cars and The Archers.
Beginning in the mid-1980s, Miles turned to writing mystery fiction. His first series, written under his own name, featured Alan Saxon, a professional golfer-turned-amateur detective. After four books, Miles's publisher did not wish to continue the series, which only resumed after a hiatus of more than a decade.
In 1988, Miles began another series, set in the theatrical world of Elizabethan London. For this series, and for most of his subsequent writing, he adopted the pseudonym Edward Marston, the name reflecting that of a real Elizabethan playwright, John Marston. The series features a fictional theatrical company, Westfield's Men, and, in particular, Nicholas Bracewell, its book-holder, a position similar to that of the modern stage manager.
Marston's next series was set during the reign of William the Conqueror; its two main characters, surveyors for Domesday Book, are Ralph Delchard, a Norman soldier, and Gervase Bret, a former novice turned lawyer, who is half Saxon and half Breton.
Miles has written three other series of historical mysteries under the Marston pseudonym: one set in Restoration London; another set in Victorian England during the 1850s against a background of the development of the railways; and the third set during the military campaigns of the Duke of Marlborough.
Under another pseudonym, that of Conrad Allen, he has written a series featuring two ship's detectives during the early 1900s, while under his own name he has written two mysteries, set in the United States, which feature a Welsh architect, Merlin Richards.
Keith Miles was chairman of the Crime Writers' Association for 1997-8. He was previously married to Rosalind Miles and is now married to another mystery writer, Judith Cutler.