Mahmood made inroads into the Hampshire top-order as Auckland bundled out the English county team for a modest 121 in 20 overs.
Mahmood then smashed an unbeaten 31-ball-55 to steer his side to a comfortable win with 33 balls to spare.
The Pakistan discard hit five sixes and four boundaries during his innings.
Auckland has thus become the first team to qualify for the main tournament, starting Saturday. The New Zealand T20 champions have won both their qualifying matches, having beaten Sialkot Stallions yesterday.
Two teams -- one each from Pool 1 and 2, will join the eight other teams for the main competition.
The result of today's match means that the final match of the Pool 1 between Sialkot Stallions and Hampshire has been reduced to a dead rubber as the two teams are now out of race for a spot in the competition.
Mahmood, who is now a British passport holder ripped apart the Hampshire top-order when he removed James Vince (11) and Jimmy Adams (0) in a space of four balls.