The ABC, where Valentine had presented TV and radio since the mid-1980s, announced the 64-year-old's passing on Thursday morning.
Valentine was best known for presenting the Afternoons show on ABC Radio Sydney for more than 20 years.
He is survived by his wife Joanne and his two children Ruby and Roy after he chose to end his life using voluntary assisted dying, his family said in a statement.
"James passed peacefully at home surrounded by his family, who adored him," Valentine's family said.
"Throughout his illness, James did it his way, which lasted all the way until the end when he made the choice to do Voluntary Assisted Dying."
Valentine was diagnosed with oesophageal cancer in March 2024 and went on leave before returning to the airwaves last year.
After later scans revealed further tumours, he made a final retirement from his extensive career in February.
"I can't think of anybody else who really lifted the mood of a city over such a long time," fellow presenter Richard Glover said when Valentine announced his retirement.
Outside of the studio, Valentine was a celebrated saxophonist who toured with The Models and recorded with well-known acts such as Kate Ceberano and Wendy Matthews.
"He has been a trusted companion for so many people, part of the rhythm of everyday life for generations of our Sydney audience," ABC Managing Director Hugh Marks said of Valentine in a statement.
"James brought warmth, wit, and humanity to radio as an exemplar of radio craft."
Fellow "music nerd" Prime Minister Anthony Albanese said there would be joy among the sadness at the news.
"He had that sense of adventure and was always really positive as well and that shone through," he said on ABC Radio.
Governor-General Sam Mostyn said it was a tragic day and she would miss not being able to pick up the phone and chat with her close friend.
"He was sharing his death with us to help us understand our mortality and how we live life better," Ms Mostyn said on ABC Radio.
"He was always the funniest fellow and the smartest and the cleverest with music."
The ABC said memorial arrangements and opportunities to honour Valentine will be shared in the coming days.
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