The American dominated the middleweight division from 1979 until his controversial defeat by Sugar Ray Leonard in April 1987.
"Today, unfortunately my beloved husband Marvelous Marvin passed away unexpectedly at his home here in New Hampshire," his wife Kay Hagler said.
Hagler fought 67 times during his 14-year professional career, winning 62.
He triumphed on 52 occasions by knockout, while he drew two and lost three times.
In September 1980, Hagler fought Britain's Alan Minter at Wembley Stadium to claim his first world titles, winning the WBA and WBC belts.
His most famous fight came in April 1985 when he fought Thomas 'Hit Man' Hearns over three brutal rounds - a classic known as 'The War'.
Hagler made 12 successful title defences until his defeat by Leonard by a split decision and he retired a year later.
More about:
