The writer, whose daughter is actress Emily Mortimer, died after a long illness, reports the BBC.
Mortimer, who began his career as a barrister in the 1940s, was a prolific author and dramatist. His first radio play was in 1957, while his last TV script was for 2001's romantic drama In Love And War.
His best-known work was the ITV drama Rumpole Of The Bailey, which starred Leo McKern in the lead role between 1978 and 1992. He also penned the ITV's 1981 11-part serial Brideshead Revisited and drama Paradise Postponed, which featured actor David Threlfall playing obnoxious MP Leslie Titmuss.
BBC head of radio drama Alison Hindell said Mortimer's death would be a "great loss" to admirers of his "wonderful works".
Melvyn Bragg, who was a neighbour of Mortimer in Oxfordshire village Turville Heath, said: "Life was encircled around that place in Turville and he was the monarch of that.
"We went to pay court to him and, to be honest, you went just to laugh and to hear the latest gossip and the latest book he'd read and 'what do you think of this and what do you think of that?'
"There was a whiff of erudition and scandal always around John and it was completely seductive and he'll be badly, badly missed."
Mortimer was knighted in 1988.