The Real Madrid winger was in unstoppable form and provided the assist as James Collins gave the hosts an early lead at the Cardiff City Stadium.
Ashley Williams inadvertently put the ball in his own net following a Johann Gudmundsson shot before the break, but Sam Vokes headed Wales back in front on 64 minutes after Bale had forced Kari Arnason to clear off the line.
And the 24-year-old rounded off his superb display by netting a fine solo effort 20 minutes from time to get Wales' preparations for the UEFA Euro 2016 qualifying campaign off to the best possible start.
Bale's threat, meanwhile, was constant and he stung the palms of Hannes Halldorsson with a dipping strike from distance 18 minutes in before Joe Allen scooped an effort over the crossbar.
But the hosts' lead was cancelled out in the 26th minute, when a tame Gudmundsson strike was deflected beyond Hennessey by the outstretched boot of captain Williams amid suspicions of offside against the visitors in the build-up.
Lars Lagerback's men dominated possession for the remainder of the half, but were lucky not to concede a penalty 10 minutes from the break when Vokes was brought down by a combination of Icelandic defenders.
Hal Robson-Kanu's mazy run nine minutes into the second half looked set to unlock the Iceland defence, but an eventual prod with the outside of the boot sent the ball wide of the right-hand post from the edge of the penalty area.
And Wales restored their lead moments later – Vokes heading into the back of the net after Arnason had done well to clear off the line from a Bale strike.
Bale's performance was complete in 70th minute when he embarked on a run that started in his own half, surged forward and stroked the ball beyond Halldorsson from just inside the box.
