Handed the captaincy ahead of the T20 World Cup in June, Afridi claimed three wickets and promoted himself in the batting order but was unable to prevent New Zealand going 1-0 up in the series.
Mitchell blasted 61 from 27 balls, following Williamson who scored 57 from 42 as New Zealand made 8-226 after being sent in on Friday - the highest-ever total against Pakistan in a T20 international.
Pakistan were in with a chance as long as Babar Azam was at the crease, but when their totem was out for 57 in the 16th over, the size of the run chase became insurmountable and the tourists were all out finally for 180 in 18 overs.
Tim Southee took 4-25 for New Zealand to become the first bowler from any nation to claim 150 wickets in T20 internationals.
Williamson's 18th half-century in T20s came in difficult circumstances. Batting first drop, he came to the crease on only the third ball of the New Zealand innings after Devon Conway was out for a first-ball duck.
He also lacked recent time in the middle after missing New Zealand's recent T20 series against Bangladesh as he continues a controlled return from the serious knee injury he suffered while playing in the Indian Premier League in April.
Williamson also was in the shadow early of Finn Allen who smashed 35 from 15 balls at the top of the order, including 24 runs — 6, 4, 4, 4, 6 — from the second over bowled by Afridi.
He batted watchfully at first but when Allen was out, Williamson steadily gained momentum to reach his half-century from 40 balls with eight boundaries.
He was twice dropped, on nine by Babar from the bowling of Abbas Afridi and on 39 by Iftikhar Ahmed off Usama Mir.
Mitchell then stepped up the pace, reaching his 50 from 22 balls. His 61 included four powerful sixes down the ground.
"I thought the partnerships we built throughout the middle with Kane and Glenn (Phillips) as well allowed us to build a platform to put the bowlers under some pressure which you always try to do at Eden Park," Mitchell said.
"Also the way the boys bowled at the end there was superb.
Saim Ayub gave Pakistan an explosive start with 27 from eight balls, featuring three sixes and two fours. But he was immensely unlucky to be run out backing up too far.
Mohammad Rizwan had to parry a delivery from Adam Milne who picked up the ball on his follow through and threw out Ayub at the non-striker's end.
Pakistan got tyo 1-62 after five overs with Mohammad Rizwan and Babar at the crease.
But Rizwan fell before the end of the power play, top-edging a ball from Tim Southee which hung for a long time in the night sky before being caught by wicketkeeper Conway.
Ish Sodhi pulled off a brilliant one-handed catch off his own bowling to dismiss Fakhar Zaman.
When Babar fell at the start of the 17th over, Pakistan's last hopes went with him.
- with AP
