The Argonauts confirmed the news on Friday night.
Thornton, who played as a defensive back, wide receiver, quarterback and running back, was part of the Blue Bombers' Grey Cup winning teams in 1961 and 1962 and made it to the CFL championship game with Winnipeg in 1965 and again with Toronto in 1971.
He was diagnosed with lung cancer earlier this year.
"We are very sorry to hear this sad news, and our organization offers our deepest condolences to Mr. Thornton's family during this time," the Blue Bombers said in an email.
As a defensive back, Thornton returned eight interceptions for touchdowns over 12 CFL seasons, including three in 1963 alone.
The Chicago native was inducted into the Bombers Hall of Fame in 1988 and played in three CFL all-star games from 1963-1971.
Thornton was an All-American standout as a quarterback, free safety and special teams performer at Northwestern University in Illinois.
He was drafted in the sixth round of the 1961 NFL draft by the Cleveland Browns — who immediately traded his rights to the St. Louis Cardinals. He was also selected by the American Football League's Dallas Texans, and was named to the Blue Bombers' negotiation list before settling on Winnipeg.