The 41-year-old Johnson floored Mack three times and referee Frank Gentile stopped the rescheduled International Boxing Federation (IBF) world title eliminator at two minutes, 21 seconds of round six.
Ex-champion Johnson pressured the younger Mack, who had made a promising start, and clinched a fine win – midway through the scheduled 12-rounder — that guarantees him a shot at the world title later this year.
“I knew he was going to box me. I just had to stay within my game plan,” said Miami-based Johnson, who was fighting for the first time since losing a points decision to American Chad Dawson in November.
The result refreshed Johnson’s career and improved his record to 50 wins, with 34 knockouts, against 13 losses and two draws and dropped Mack’s log to 28 wins (17 knockouts) against three losses and two draws.
The bout was originally slated as the co-main event for the cancelled January 30 Andre Berto-Shane Mosley card in Las Vegas.
After a competitive first round, the 30-year-old Mack appeared to win the second round by staying on the outside and boxing effectively as he tried to frustrate the slower Johnson.
Determined to prove he still has championship mettle, the IBF’s No.3 ranked Johnson pressed the accelerator during the third round and Mack struggled to keep pace.
Mack scored some good shots in the fifth but Johnson grabbed control in the sixth with a flurry of stinging blows.
Former IBF champion Johnson, after setting up Mack with left jabs, exploded with a big right hand that dropped Mack, who fell again to another right hand soon after.
Mack, ranked No.7 by the IBF, beat the count but remained groggy and when Johnson floored him with a left hook near the end of the round, referee Frank Gentile appropriately stopped the fight.
“I felt tight the first couple of rounds but everything started coming together after that,” said Johnson, who is originally from the central Jamaican parish of Clarendon.
With the win, the 2004 World Fighter of the Year is now the IBF’s mandatory challenger to fight champion Tavoris Cloud, probably in May.
