Landen was born at Margate, Kent and educated at King's School, Rochester. He made his television debut in 1959 as the adult Pip in an adaptation of Great Expectations and made his film debut in 1960, with a walk-on part in The League of Gentlemen.
He first became well known during the 1960s when he starred in the TV series Mickey Dunne and The Mask of Janus, which was renamed The Spies in later series. He also had a parallel career as a stage actor, including as Richard Dazzle in the RSC's 1970 production of London Assurance; and was Olivier Award nominated for his role in James Saunders's play Bodies in the West End in 1979.
His film roles include appearances in Operation Snatch (1962), A Jolly Bad Fellow (1964), Rasputin, the Mad Monk (1966), Mosquito Squadron (1969), Every Home Should Have One (1970), Young Winston (1972), Digby, the Biggest Dog in the World (1973), International Velvet (1978), Morons from Outer Space (1985) and both The Buccaneers and The Steal in 1995.
On radio, he appeared in the 1973 BBC Radio 4 adaptation of The Foundation Trilogy, as Dr. Watson in the 1974 adaptation of A Study in Scarlet, and as General Bel Riose and Art Gordo in the 1976 adaptation of Jim Eldridge's novel Down Payment on Death. He portrayed Rupert Purvis in the 1982 production of Tom Stoppard's play The Dog It Was That Died, and played the urbane Ambassador McKenzie in BBC Radio 4 series of Flying the Flag.
In 1977 Landen starred in his own situation comedy, Devenish, playing a Basil Fawlty-type character in a Reggie Perrin-type situation, designing board games. In 1980 he starred as Barty in the television series Pig in the Middle with Liza Goddard.
In 1984, Landen achieved a memorable performance as Jean-Martin Charcot in the TV series Freud. In 1985, he and his wife, Actress Jennifer Daniel, wrote The True Story of H.P. Sauce.
In 1987 he played the lead in a BBC TV production of What the Butler Saw, playing Dr Prentice in a production also featuring Prunella Scales, Timothy West and Bryan Pringle.
In 1989 he made a guest appearance in Doctor Who, playing Dr. Judson, a wheelchair-using genius, in the serial The Curse of Fenric.
In 1992, Landen provided the voice of the arch villain Mr. Tod in the BBC/Fuji Television children's animated series The World of Peter Rabbit and Friends.
Dinsdale Landen died at his home in South Creake, Norfolk, on 29 December 2003 after becoming ill with pneumonia. He had been diagnosed with oral cancer several years before his death, but was in remission at the time. He was married to the Actress Jennifer Daniel.
