Steward trained 41 world champions, who accumulated a record of 34-2-1 in title fights, the Los Angeles Times reported.
Victoria Kirton, Steward's executive assistant, said he died Thursday in Chicago. The cause of death was not disclosed.
The trainer had worked as a boxing analyst for HBO since 2001.
Steward, born July 7, 1944, in Bottom Creek, W.Va., moved with his mother to Detroit after his parents' divorce.
He won a Golden gloves title, but gave up a boxing career to support his family as an electrician.
Steward began to train boxers at Kronk's Gym, which he would later build into one of the best-known boxing centers in the world,and where he would train some of the best fighters in the United States.
HBO's lead boxing announcer, Jim Lampley, said the depth of Steward's knowledge about the field was "unsurpassed."
Information about survivors was not immediately available.