Walker began his radio career in 1966 on Swinging Radio England, an offshore pirate station, before moving to Radio Caroline.
He joined Radio 1 in 1969, and joined Radio 2 in 1997 where he has remained ever since.
Among those paying tribute to him was Helen Thomas, head of Radio 2, who called him a “broadcasting legend”.
“Everyone at Radio 2, and I’m sure his millions of listeners, would like to thank Johnnie for entertaining us for so many decades with his brilliantly crafted shows.” she said.
“He will be much missed on the airwaves and by his listeners, who have adored listening to his shows over the years.”
Harris, who will continue to host Radio 2’s The Country Show, said he was “proud and honoured” to be taking over the helm of Sound of the 70s from Walker.
He added: “I will do everything I can to maintain his legacy and curate the programme with the very best music from that incredible decade.”
Keaveny, who has hosted various shows on Radio 6 Music and has sat in for Walker on The Rock Show before, called him one of his “ultimate radio heroes”.
“I am not sure how I am going to get through the week without hearing him, and his place in radio legend is assured,” he said.
Walker’s last episode of The Rock Show will air on 25 October, and his final episode of Sounds of the 70s will air two days later.
