Born in Salford, and educated at Manchester Grammar School and Salford University, Powell took up acting while an undergraduate. He had a small role in the original version of The Italian Job playing one of the drivers, but had to wait a few years for his first success, playing scientist Toby Wren in the BBC's sci-fi series, Doomwatch in 1970.

Having been at his request killed off in the last episode of the original series, Powell became a pin-up and a household name, following up with starring roles in several BBC serials, including Sentimental Education and Jude the Obscure.

For several years, Powell continued as a television regular, with occasional forays into film, notably as Mahler in 1974. He also notably played Captain Walker in the 1975 Ken Russell film version of Tommy. His role had no lines at all and apart from a few early scenes during the overture with Ann-Margret, he is primarily seen through the mind of his son as played by Barry Winch (Young Tommy) and Roger Daltrey. In one of those scenes Captain Walker is shown in a crucifixion pose.

He then played Jesus Christ in the series Jesus of Nazareth following a successful second audition with Franco Zeffirelli. The series had an all star cast, such as Sir Laurence Olivier, Christopher Plummer, Rod Steiger and James Mason. For this role, Powell was nominated for a BAFTA award, and collected the TV Times Best Actor award for the same performance.

In 1975, Powell married his girlfriend, the Pan's People dancer Babs Lord. This happened quickly and quietly, partly due to the fact he was about to start filming for Jesus of Nazareth, and partly to overcome problems if she flew out to see him on location in Morocco. Both felt it would be easier if they were married.

On 23 November 1977, they had their son, Barney, followed in 1979 by a daughter, Kate.

In 1978, Powell took the leading role of Richard Hannay in the third film version of The Thirty-Nine Steps. It met with modest success, and critics compared Powell's portrayal of John Buchan's character favourably with his predecessors.

His characterisation did indeed prove to be enduring, as almost ten years later a television series entitled simply Hannay appeared with Powell back in the role, (although the Buchan short stories on which the series was based were set in an earlier period than The Thirty-Nine Steps). Hannay ran for two seasons.

In 1980 Powell appeared in the film Harlequin playing the Harlequin of the title who seems to have the power to cure the son of a powerful politician. For this performance, he won the Best Actor Award at the Paris Film Festival. In 1982, he won Best Actor at the Venice Film Festival for his role in Imperativ.

Powell then agreed to a request from his old friend and golf partner, comedian Jasper Carrott, taking the part of an incompetent detective in a succession of sketches that formed part of Carrott's television series. The Detectives was so popular that it was turned into a sitcom, Powell's first and only venture into this genre.

In 1986, Powell narrated and co-starred in William C. Faure's popular miniseries Shaka Zulu...with soccer legend Henry Cele in the title role.

In 1992, Powell starred in the New Zealand World War I film Chunuk Bair, as Sgt Maj Frank Smith.

Nowadays Powell appears in person less often, but his distinctive voice is frequently heard on voice-overs, advertisements, and as a narrator of television programmes such as Great Crimes and Trials.

He has also narrated many audio books including The Thirty Nine Steps, abridged versions of many of Alan Garner's books, and several abridged novels for 'The Talking Classics Collection'.

Powell has also lent his voice to musical works, such as David Bedford's album The Rime of the Ancient Mariner, or the 2002 rock opera The Hound of the Baskervilles, by Clive Nolan and Oliver Wakeman, where he played the role of John Watson. He also made use of his narrating skills on a rock album by Rick Wakeman called The Cost of Living.