A late flurry from Mahmudullah has boosted Bangladesh to 8-141 in the first Twenty20 global against New Zealand at McLean Park in Napier on Tuesday.
But Williamson (73 off 55 balls) then paired with Colin de Grandhomme (41 off 22 balls) for an unbroken 81-run partnerhip, off just 47 deliveries, to coast to the win with two overs to spare.
Bangladesh scored 141 for 8 wickets in 20 overs after skipper Mashrafe Mortaza won toss and chose to bat first at McLean Park. New Zealand eventually finished on 143 for 4 in 18 overs.
Opting to bat first, Bangladesh were rocked early as Imrul Kayes couldn't keep his bat away from a Matt Henry delivery that was angled across the left-hander.
Rubel Hossain, Shakib Al Hasan and Mustafizur Rahman each picked up a wicket for Bangladesh. Mustafizur Rahman too struck in his very first over to remove Colin Munro for a second-ball duck.
Ferguson, who made his one-day worldwide debut for New Zealand earlier this season, made his mark in his T20 debut when he dismissed Sabbir Rahman and Soumya Sarkar with the first two balls of his opening over.
Tom Bruce made his mark on debut when he took a catch to dismiss Bangladesh opener Tamim Iqbal for 11 off his Central Districts teammate and fellow debutant Ben Wheeler.
In came Mahmudullah, who had failed to fire in the one-day series with scores of 0, 1 and 3, but he held together the visitors' below-par innings with a 45-ball fifty before falling for 52, also to Ferguson, in the final over. However, Colin de Grandhomme made Shakib (14) caught by Santner in the 11th over to erase the half of the Bangladesh batsmen for only 67. In a game where bowlers from either side dominated most parts, Williamson nullified Mahmudullah's efforts to ensure the hosts kick-started 2017 on a bright note with a six-wicket victory.
The match looked as if it wasn't going to go the distance after New Zealand's opening three bowlers including debutants Ben Wheeler and Lockie Ferguson tore through the Bangladeshi top order.