Monzon began his professional boxing career in Argentina in 1963. He was the Argentine and South American middleweight champion when he went to Rome and won the world middleweight title by knocking out Italian Nino Benvenuti (Benvenuti, Nino) in the 12th round on Nov. 7, 1970. Fighting mainly in Europe, Monzon successfully defended his title 14 times against 11 opponents before retiring in 1977. He won 87 (some sources say 89) fights, 59 of those wins coming on knockouts, and had 9 draws, 3 losses (all on judges' decisions early in his career), and 1 no contest.

In 1988 Monzon was sentenced to 11 years in prison for killing Alicia Muniz, his longtime companion and mother of his son. Returning to prison after a weekend furlough, Monzon was fatally injured when the car he was driving went out of control and plunged down an embankment. He was inducted into the International Boxing Hall of Fame in 1990.