Scott LeDoux, a former WBC heavyweight title contender, has died aged 62 from amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease.
The American lost to Larry Holmes on a technical knockout in July 1980 when challenging for the WBC belt.
LeDoux also fought George Foreman and Frank Bruno in a 10-year professional career that ended in 1983.
Nicknamed 'The Fighting Frenchman', LeDoux won 33 of his 50 pro fights, 22 by knockout.
LeDoux's final fight was a third-round stoppage by Briton Bruno at Wembley Arena in May 1983.
After boxing he entered politics and was a commissioner in Anoka County in his home state of Minnesota, until he stepped down in 2010 because of his declining health.
Rhonda Sivarajah, the chair of Anoka County board of commissioners said: "He was a big man with a gentle spirit. I can only imagine the sadness at losing someone so special after such a valiant battle."
LeDoux was the son of a miner who grew up in northern Minnesota. His first professional fight was in February 1974 when he knocked out Arthur Pullins in the third round in Minneapolis.
He won his first 12 fights before losing to Roy Wallace in March 1975. He fought Foreman in Utica, New York in August 1976, losing in the third round.
He earned draws with future world champion Leon Spinks and Ken Norton before being defeated by Holmes in Bloomington, Minnesota.
LeDoux also fought boxing great Muhammad Ali in a five-round exhibition bout.
ALS, a form of motor neurone disease, is also known as Lou Gehrig's disease after the New York Yankees baseball legend whose career and life were cut short by it.