Word of his death came from Harvey Goldberg, a music producer who is a friend of the musician.
Phillips and the other members of the folk group he founded -The Mamas & the Papas - took the world by storm in the 1960s and made hit after hit, many of which are still well-known and performed today.
The best known of their hits include California Dreamin', I Saw Her Again Last Night, and Monday, Monday.
"Very few songwriters can write combination of sophisticated melody and lyric and still make the song be immediately recognizable. John could do this with ease and just as warm as his personality felt when you sat in a room with him, it was felt in his music," said Goldberg, in an interview with CBS Radio News.
He adds that while Phillips "may have had his ups and downs, he was still a kind and gentle soul."
Phillips' accomplishments include organizing, with Lou Adler, the 1967 Monterey Pop Festival, the first large-scale rock festival.
He is the father of actress McKenzie Phillips, who has gone public with her story of what it was like to grow up in her father's house, which was filled with drugs and was often the scene of parties focused on drugs.
Later, they both kicked the drug habit and at one point made appearances together, talking about the perils of drug addiction.
Phillips never stopped making music and recently completed an album with Keith Richards and Mick Jagger that the three had started to make 25 years ago.
Just before going into the hospital, Phillips had finished making another album - of new music.
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