Although Shaw is not Classified as a film actor he has had several roles over the years. His first film role was an Irish communist in Love on the Dole (1966). Better known is his '70s role of Banquo in Roman Polanski's Macbeth. He has also been seen as an undercover Second World War operative in Operation Daybreak; a singing and dancing Futuristic Magician "Zax" in Facelift; the role of Rachid in the 1973 film The Golden Voyage of Sinbad, and a wanted villain leading a life on the run in a circus troop in Ladder of Swords.

Shaw began television work in 1967. Parts in one-off plays for Granada Television led to his playing hippy student Robert Croft, Lucile Hewitt's boyfriend, in Coronation Street. Another early role was booze and football-loving Welsh medical student Huw Evans in the television comedy series Doctor in the House. Huw Evans later returned in the Doctor at Large episode "Mother and Father Doing Well" as a nervous expectant father.

Shaw took key roles in the first revival of Look Back in Anger (Royal Court/Criterion, 1968); in the National Theatre's Saturday, Sunday, Monday opposite Laurence Olivier (1973); and in A Streetcar Named Desire presented by the Piccadilly Theatre in 1974. He later acknowledged the role of Stanley Kowalski in 'Streetcar' as a point of breakthrough in his career.

In 1971, Shaw became a follower of Charan Singh, of the Sant Mat religion.

Among several voiceovers and appearances, in 1974, Shaw starred in a three-minute advertisement for the Mk II Ford Capri and in 1987, a TV advert for the Vauxhall Cavalier.

In the 1980s, Shaw played Elvis Presley in Alan Bleasdale's critically acclaimed Are You Lonesome Tonight?. It told the story of Presley's last few hours. After a long run in London, the production visited Sydney, Melbourne and Adelaide in Australia.

In 1983, Shaw played Robert Falcon Scott in The Last Place on Earth. The series was filmed at Frobisher Bay near the city of Iqaluit on Baffin Island, Canada. In interview at the time, Shaw commented that he generally responded well to the testing physical conditions, particularly when they enhanced the reality of the scene. In the same year he played Sir Henry Baskerville in The Hound of the Baskervilles, an adaptation of the novel by Sir Arthur Conan Doyle. He acted opposite Ian Richardson's Sherlock Holmes and Donald Churchill's Dr. Watson.

Shaw won two awards in 1996 for his performance as Lord Goring in the Broadway production of An Ideal Husband and was nominated for a third:

In 2001, he took the title role in the BBC drama Judge John Deed. The character gave an editorial voice to the television Writer and Producer G.F. Newman's ideas about lifestyle choices such as vegetarianism and alternative Medicine as well as issues of social justice. One episode about the safety of the MMR vaccine was banned.

Between seasons of Judge John Deed, Shaw took the role of poetic bespectacled forensic detective Adam Dalgliesh in P.D. James's Death in Holy Orders in 2003 and The Murder Room in 2005. After the sixth season of Judge John Deed had been filmed, Shaw appeared in the series Apparitions broadcast by the BBC in 2008. This was Shaw's first project as executive Director.

In December 2006, Shaw presented the six-part Discovery Channel Real Time TV series Martin Shaw: Aviators, produced by Twofour, which followed the two-year restoration of his Boeing Stearman biplane after it was crashed by another pilot at Old Buckenham airfield in Norfolk. Shaw fulfilled a lifetime ambition to take the controls of a Spitfire (owned by Maurice Bayliss) and, though take-off was not permitted, he also powered an English Electric Lightning (owned by Russell Carpenter) to 150 mph in three seconds along the runway at Cranfield Airport. Shaw also compared notes with the nonagenarian builder and developer of the modern autogyro, Wing Cdr. Ken Wallis.

From 2007 he played the title role in the BBC TV series Inspector George Gently with co-star Lee Ingleby.

On 18 August 2010, Shaw collapsed during the first act of the matinee showing of A Country Girl at Shrewsbury's Theatre Severn. His agent, Roger Charteris, said he had been suffering from cracked ribs and was taking antibiotics for a severe chest infection. An understudy went on in his place.

In March 2012, he also announced that he would become the official patron to the community organisation Stop Norwich UrBanisation or SNUB, whose aim is to protect Norfolk's countryside from overdevelopment and excessive urbanisation. In the press release, he stated that he was "simply furious and upset by your plight and that of all of us who wish to live in quiet and peace. I will be your Patron and keep fighting".

Shaw appeared with Future co-star Lewis Collins in an episode of The New Avengers. Both played the roles of terrorists. Shaw portrayed Ray Doyle ("Agent 4–5") in the British television series The Professionals (1977–1981), opposite Collins. Shaw played another law-enforcement role in the 1990s ITV production The Chief.

On 9 May 2015 he recited For the Fallen at VE Day 70: A Party to Remember in Horse Guards Parade, London which was broadcast on BBC 1.

In 2016 Shaw toured and hit the West End again with a lively production of Hobson's Choice at the Vaudeville. After completing filming the final episode of George Gently, Shaw again toured in 2017 with the U.K. premier of Gore Vidal's 1960 political piece: The Best Man. Shaw played the part of william Russell, former US Secretary of State.