Lord Rix, who was the president of Mencap since 1980, was also the entertainer behind a hit run of Whitehall farces in London in the 1950s and 1960s.
"Lord Rix was a beloved colleague and friend to so many people with a learning disability and their families," Mencap chief executive Jan Tregelles said. "His passion, zeal and humour will be sorely missed. His tireless campaigning has perhaps done more to improve the lives of people with a learning disability than any other."
Rix announced in 2016 that he was terminally ill and called for the legalisation of voluntary euthanasia for those dying in severe pain.
"Unhappily, my body seems to be constructed in such a way that it keeps me alive in great discomfort when all I want is to be allowed to slip into a sleep, peacefully, legally, and without any threat to the medical or nursing profession," he said in a letter that attempted to explain to his fellow peers in the House of Lords why he no longer opposed assisted dying legislation.
"Only with a legal euthanasia bill will the many people who find themselves in the same situation as me be able to slip away peacefully in their sleep instead of dreading the night."
Offstage, aside from his charitable work, Rix chaired the Arts Council of Great Britain's drama panel from 1986 to 1993 and was responsible for opening up grant funding for a wider mix of performers, more involvement for women and more funding for ethnic minority theatre companies.
Rix, who is perhaps still best known for his roles as crooked bookies or harassed civil servants, was married for 64 years to actor and fellow campaigner Elspet Gray, who died in 2013. They appeared together on stage and screen several times during their long marriage.
They began to take an active interest in the problems faced by people with learning disabilities when their daughter Shelley, the eldest of four, was born with Down's syndrome in 1951. At the time the condition was still referred to as mongolism and there was little support on offer. Rix became involved with Mencap (then the National Society for Mentally Handicapped Children and Adults) to try to change this.
The actor, who was born in east Yorkshire, first found acting work at the age of 18 with Donald Wolfit's Royal Shakespeare Company while on deferred service from the Royal Air Force. Only a few months later, he played Sebastian in Twelfth Night in the West End. When his military deferment was extended, he gained more regular stage experience with the White Rose Players in Harrogate. After serving in the RAF, he ended up as a volunteer Bevin Boy, working in Doncaster's coal mines.
Rix made his name as an actor-manager after appearing on tour with Gray in the farce Reluctant Heroes by Colin Morris. He moved his company of comic performers, which included actors Leo Franklyn, Derek Royle, Terry Scott, Andrew Sachs and Rix's sister, Sheila Mercier, who later played Annie Sugden in Emmerdale Farm, into the Whitehall Theatre in 1950, where he enjoyed a long period of popular success, later moving to the Garrick Theatre. Key farces included Dry Rot by John Chapman and Chase Me, Comrade by Ray Cooney, and although this brand of comedy drew big audiences and enjoyed record-breaking runs Rix said he was resigned to not winning much recognition from the serious sector of the profession.
In 1980 theatre critic Michael Coveney expressed regret about critics' attitude to Whitehall farces. "A tradition of critical snobbery has grown up around these plays," he wrote, "partly because they were so blatantly popular, but chiefly because of our conviction that farce, unless written by a Frenchman, is an inferior theatrical species. Once the National Theatre has done its duty by Priestley and Rattigan and others teetering on the brink of theatrical respectability, I suggest they employ Mr Rix ... to investigate the ignored riches of English farce between Travers and Ayckbourn."
One-off TV comedies put on by Rix regularly drew audiences of 15 million or more, but few recordings survive. The actor was made a life peer in 1992. His daughter Shelley died in 2005. His sons are Jonathan and Jamie, a producer and a children's author. His surviving daughter is actor Louisa Rix, with whom he presented Let's Go for the BBC in the late 1970s and early 80s. It was the first television programme designed for people with a learning disability.
Brian Rix, Baron Rix died in August 2016 in England.