A slick, hard-hitting technician in the ring, McCallum was known for his exceptionally durable chin and toughness, and was never stopped in any of his five losses.

He first became a world champion in 1984 by defeating Sean Mannion to win the vacant WBA super welterweight title.

Curry was ahead on all three scorecards going into the fifth round when McCallum knocked him out with what some have called a "perfect" left hook.

In 1989, McCallum defeated Herol Graham by a split decision[6] to win the then-vacant WBA middleweight title (which had been stripped from Kalambay for signing to face IBF champion Michael Nunn).

At 40 years of age, he attempted to regain the vacant Interim WBC title against Roy Jones Jr. in December 1996, but lost by a wide decision.

In his last fight, McCallum lost a rubber match to James Toney via a unanimous decision in a cruiserweight bout.