Fleetwood wrote on his Facebook that "Danny was a huge force in our early years", and his love of the blues led him into the band back in 1968, fifty years ago.
The musician passed away in London, England on Friday, though no cause of death has been given.
Thank you, Danny Kirwan.
As a teenager, Kirwan's band Boilerhouse supported Fleetwood Mac at venues in London before he was invited to join the group itself in 1968.
His continued success didn't suffer, however, as he was even inducted into the Rock and Roll Hall of Fame in 1998 for his work as part of Fleetwood Mac.More news: Trump adviser says Trudeau 'stabbed us in the back' at G7 summit
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Kirwan released three solo albums in the years following his release from Fleetwood Mac, but disappeared from the music industry around 1980. "You will forever be missed!" the singer-songwriter explained.
Divorced, Kirwan is survived by a son.
After Blues Jam At Chess (also known as Fleetwood Mac In Chicago), and by now fully integrated into the band. He then played on their third album, Then Play On, released in September 1969, which reached #6 on the United Kingdom charts.
His first recorded work reached the US on the 1969 compilation album "English Rose", which included the band's chart-topping United Kingdom instrumental "Albatross" and its B side, Kirwan's composition "Jigsaw Puzzle Blues". For their sixth album, 1972's Bare Trees, Green and Spencer were long gone, replaced by Bob Welch and Christine McVie.
But Kirwan struggled with alcoholism, and he was eventually dismissed from the group in 1972. Then 1971's Future Games opened with his memorable, achingly atmospheric 'Woman Of 1000 Years, ' among three Kirwan songs on the record.
Kirwan, who was also a songwriter, was sacked in 1972 according to the Rock & Roll Hall of Fame's website.