No cause of death was given Sunday (Jan. 7), but Thomas disclosed in 2014 that he had been diagnosed with prostate cancer.
Born in 1941, Thomas founded The Moody Blues in 1964 with fellow musicians including Mike Pinder and Denny Laine.
The band soon swapped blues roots for a more orchestral sound that came to be called progressive rock. Thomas's flute solo was a key ingredient on one of its biggest hits, "Nights in White Satin."
The band is due to be inducted into the Rock and Roll Hall of Fame in Cleveland, Ohio in April.