Born in London, Ashe spent several years in Canada growing up, graduating from the University of British Columbia, Vancouver, before continuing at Cambridge.

Many of his historical books are centered on factual analysis of the Arthurian legend, and the archaeological past of King Arthur, beginning with his King Arthur's Avalon: The Story of Glastonbury, in 1957. The book was inspired by what Ashe had read in G. K. Chesterton's Short History of England.

He is a major proponent of the theories that the historical King Arthur was Riothamus, presented in an article in Speculum, April 1981, and expanded in The Discovery of King Arthur (1985), The Landscape of King Arthur (1987), and in various further articles. His fresh idea was to scrutinize Arthur's foreign campaigns in Geoffrey of Monmouth's account and take the material seriously, concluding that, though the legendary Arthur is a composite figure, the career of Riothamus seems to underlie at least a major portion of Geoffrey's account, for which Ashe adduces passages in a Breton text and several chronicles.

Ashe has also helped demonstrate, through acting as secretary to a dig undertaken by Dr. Ralegh Radford in 1966-70, that Cadbury Castle in Somerset, identified as Camelot by the sixteenth-century antiquary John Leland, was actually refortified in the latter part of the fifth century, in works as yet unparalleled elsewhere in Britain at the time. Ashe's point is that when Leland picked out this hill as Camelot, he picked what seems to be the most plausible candidate; yet even an archaeologist could not have guessed that the fifth century fortification was embedded in the earthworks, just by looking without digging.

"I would say there must have been a tradition about the hill and its powerful overlord, handed down from the Dark Ages", Ashe has said, and added "In the film of the musical Camelot, you have a brief glimpse of a map of Britain, and Camelot is in Somerset. It's there because I told Warner Brothers to put it there. That is my one contribution to Hollywood." He has offered later mentions of Artoriani or "Arthur's men," a group of soldiers sharing Arthur's name (as has happened to other historical generals of the age) that survived his death, as possible basis for the legendary Knights of the Round Table.

While Dr. Raleigh Radford played a huge role in the archaeology of Somerset and other places, the excavations at South Cadbury between 1966-1970 were carried out under the direction of Leslie Alcock. For an excellent account of the digs and findings, see: Was This Camelot? Excavations at Cadbury Castle 1966-70 by Alcock.

Was This Camelot? Excavations at Cadbury Castle 1966-70; Leslie Alcock, Thames and Hudson, Publishers, ISBN 8128-1595-x