Ackland was born in North Kensington, London, the son of Ruth (Izod) and Sydney Norman Ackland. He was trained by Elsie Fogerty at the Central School of Speech and Drama, then based at the Royal Albert Hall, London. Ackland and his wife, the former Rosemary Kirkcaldy, were married on 18 August 1951 when Ackland was 23 and she 22. She was an actress and Ackland wooed her when they appeared on stage together in the town of Pitlochry in Scotland. The couple struggled initially as Ackland's acting career was in its infancy. They moved to Kenya, where Ackland managed a tea plantation for six months, but, deciding it was too dangerous, they moved to Cape Town, South Africa. Though they both obtained steady acting jobs in South Africa, after two years, they returned to England in 1957.
Ackland joined the Old Vic, appearing alongside other notable actors including Maggie Smith, Judi Dench and Tom Courtenay. Ackland worked steadily in television and film in the 1960s and 70s. He worked opposite Alec Guinness in the 1979 television serial Tinker Tailor Soldier Spy, playing sporting journalist and intermittent British espionage operative Jerry Westerby, and his career advanced through the 1980s with important parts in such films as The Sicilian, Lethal Weapon 2, The Hunt for Red October and White Mischief. Ackland also appeared in Passion of Mind with Demi Moore and the two-part TV serial Hogfather based on Terry Pratchett's Discworld. He played C. S. Lewis in the television version of Shadowlands before it was adapted into a stage play starring Nigel Hawthorne and then a theatrical film with Anthony Hopkins in the same role.
His stage roles included creating the role of Juan Perón in Tim Rice and Andrew Lloyd Webber's musical Evita opposite Elaine Paige. He also starred in the London production of Stephen Sondheim's and Hugh Wheeler's A Little Night Music with Jean Simmons and Hermione Gingold, performing on the RCA Victor original London cast album.
Ackland appears in the Pet Shop Boys' 1987 film It Couldn't Happen Here, and in the video for their version of the song Always on My Mind, which was taken from the film. Several years later, he claimed in an interview with the Radio Times that he appeared with the band purely because his grandchildren liked their music.
Ackland, in a 2001 interview with the BBC, admitting to being forced to make "awful films" due to being a workaholic, said that he "regretted" appearing in Bill & Ted's Bogus Journey and the Pet Shop Boys music video, while lambasting former co-star Demi Moore as "not very bright or talented".
In 2007 Ackland narrated and provided the voice for the Robert Garofalo biography film/documentary on the notorious occultist Aleister Crowley, titled In Search of the Great Beast 666 that was released on DVD.
Also in 2007, Ackland appeared in the film How About You opposite Vanessa Redgrave, portraying a recovering alcoholic living in a residential home after being forced to retire and losing his wife to cancer.
In 2008 he returned to the small screen as Sir Freddy Butler, a much married baronet, in the popular ITV1 whodunnit show Midsomer Murders. The episode, entitled Vixens Run also featured veteran actress Siân Phillips.
In September 2013 Jonathan Miller directed a Gala Performance of William Shakespeare's King Lear at the Old Vic in London. Ackland played Lear.
Ackland and his wife were married for 51 years. They had seven children and, as of May 2006, 32 grandchildren. Despite his filming taking him to far-flung locations, Ackland said they never spent a night apart. In 1963, their house in Barnes caught fire. Rosemary Ackland managed to save their five children but broke her back when jumping from the bedroom window to safety. She was told she would lose the baby she was carrying and would never walk again, but later gave birth and, after two years in Stoke Mandeville Hospital, walked again.
Their eldest son, Paul, died of a heroin overdose in 1982, aged 29. In 2000 Rosemary Ackland was diagnosed with motor neurone disease; she died on 25 July 2002.
Ackland edited and published his wife's diaries in 2009 under the title My Better Half and Me: A love affair that lasted fifty years.