The Crystal Palace man hit the bar after just 15 seconds and got Wales' second goal late on in a performance where his trickery and guile constantly troubled the Iceland defence.
Substitutes Wes Burn and Robert Ogleby got the other goals for the hosts, but it was Williams who caught the eye of his manager.
"Jonathan Williams is a top-class player and enhances the side no end," he said.
"I am delighted for him, it was a great performance and he got his goal.
"This level is about producing players who can give Chris Coleman a headache and if his form means we need to give another young man a chance in the Under-21s then that would be fantastic."
The victory gives Wales a valuable morale boost ahead of the start of their 2015 European Championship qualifiers next month, which sees them in the same group as England.
"We have only been together since Sunday but this was very positive," said Geraint Williams.
"We have had to get our philosophy into the players and they have been very receptive.
"It was evident we had a good pattern of play and the second half where the subs came on and changed the game was excellent.
"We looked like we knew what we were doing and the players know what we want. The result is pleasing but the main thing was the players having that understanding."