The guitarist died of a heart attack in his sleep late Sunday, said a statement released by AristoMedia.
He was still a teen when he joined The Outlaws in the late 1960s. Signed by Arista Records, the band released 12 albums, including "The Outlaws," "Bring It Back Alive" and "Ghost Riders in the Sky."
When The Outlaws broke up, Thomasson joined Lynryd Skynyrd, touring and recording with that band for nine years until 2005, when he reformed The Outlaws.
The Outlaws recently toured the United States with The Charlie Daniels Band and The Marshall Tucker Band and played a festival in Amsterdam with Aerosmith, Scorpions and Tesla.
The band has completed a new album, "Once an Outlaw," which hasn't been released.
A private memorial service for family only is scheduled, with a public memorial service to take place at a later date.