On his personal Facebook page, Mick wrote: 'Today was greeted by the sad news of the passing of Danny Kirwan in London, England.
The tribute did not indicate a cause of death.
Thank you, Danny Kirwan.
Fleetwood said it was Kiran's "love for the blues" that led him to the band.
Kirwan was a member of the band from 1968 to 1972 and played on five of the band's early albums: Then Play On (1969), Blues Jam at Chess (1969), Kiln House (1970), Future Games (1971) and Bare Trees (1972).
In the band's Facebook post, Fleetwood hailed Kirwan's legacy and said it "will forever live on in the music he wrote and played so beautifully as a part of the foundation of Fleetwood Mac". You will forever be missed!,' he added.
Kirwan was born in the South London neighborhood of Brixton on May 13, 1950.
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". It cites Kirwan's Bare Trees' track "Sunny Side of Heaven" as "one of the 10 best Mac songs ever recorded".
Later that year, Kirwan was sacked from the band, amid bad behavior and alcoholism.
According to Variety, no details surrounding his death are immediately available.
Kirwan joined following the release of the band's second album, Mr Wonderful, with his first appearance on the No 1 single Albatross.
Kirwan was sacked by Fleetwood, who became acting manager in the band, in 1972 due to continued confrontations regarding his alcoholism.
Kirwan was among the members of the band inducted into the Rock & Roll Hall of Fame in 1998, though he didn't attend the induction ceremony, Rolling Stone magazine noted.