Myles suffered the heart attack Wednesday and was on life support before his death late Thursday night, the Cowboys reported, citing reports.
The death initially was announced on the University of Florida football program's official Twitter account, which listed the cause of death as a massive stroke.
Myles, a first-team All-Southeastern Conference selection as a Gators senior, was drafted in 1990 by the Cowboys and played six seasons with the team. He was part of three Super Bowl championship teams and made 11 career starts, all during the 1995 season.
Myles was one of three men indicted in March on federal mortgage-fraud charges, the Orlando Sentinel reported.