Thomas Miller was the bassist of Symphony X prior to the entrance of Andy DeLuca, and soon after, Michael Lepond. Miller left the band along with drummer Thomas Walling for their unwillingness to tour for the support of Symphony X's fourth studio album, Twilight in Olympus.
Miller has been a composer of the Symphony X's studio albums The Damnation Game, The Divine Wings of Tragedy, and Twilight in Olympus. Miller wrote the lyrics to the songs "Of Sins and Shadows," "The Eyes of Medusa," "Candlelight Fantasia," and part of "The Divine Wings of Tragedy" from the album The Divine Wings of Tragedy. He has also contributed to over forty other albums from outside projects, including the album Filled with Your Glory by Calling Levi.
Miller, along with Symphony X keyboardist Michael Pinnella, was praised on his performance on The Divine Wings of Tragedy by Allmusic as being "equally competent on their respective instruments" to guitarist Michael Romeo, who was praised for his "pyrotechnic displays."