He was known for his acting roles in movies such as Henry Salt in the 1971 film Willy Wonka & the Chocolate Factory, Algernon in The Beatles' Help!

[1] Kinnear's acting career began in 1955, playing Albert in The Young in Heart, at the repertory theatre, Newquay.

His best-known films are those he made with director and close friend Richard Lester:[1] Help!, A Funny Thing Happened On the Way to the Forum, How I Won the War, The Bed Sitting Room, Juggernaut[1] and the Musketeer series of the 1970s and 1980s.

His final completed roles were in A Man for All Seasons (1988) a made-for-television film directed by and starring Charlton Heston, John Gielgud and Vanessa Redgrave, as a patient in the BBC One hospital drama Casualty, and a voice role as Mump in The Princess and the Goblin, which was released in 1991, three years after his sudden death in September 1988.

[8] In Oct 1988 Radio 4 first broadcast The T Machine, an episode of the comedy series The Fall of the Mausoleum Club in which he played the lead character, Mr Tilly.

[10] Their elder daughter, Karina, was a quadriplegic and had profound learning difficulties;[11] she died in May 2020 from coronavirus, [10] and was buried very close to her father.

On 19 September 1988, Kinnear fell from a horse during the making of The Return of the Musketeers in Toledo, Spain, sustaining a broken pelvis and internal bleeding.

[13] In May 1994, the Roy Kinnear Trust, which was inspired by his daughter, Karina (1972–2020), was founded to help improve the life of young adults with physical and mental disabilities.