Les Bleus ended a 10-year wait for a victory in Cardiff to outline their title credentials in a thriller at a raucous Principality Stadium on Sunday (AEDT).
Five-eighth Ntamack scored a crucial intercept try in the second half after first-half scores from Anthony Bouthier and Paul Willemse.
Dan Biggar's 18-point haul for Wales was in vain as the defending champion suffered back-to-back defeats to leave its hopes of retaining the title in tatters.
France went on to complete a clean sweep after its previous win in Wales in 2010, a year that also marks its most recent Six Nations tournament victory.
Ntamack, who was named man of the match after scoring 17 points and showing maturity way beyond his years, says France is not getting ahead of itself in what has been a dream start to the Fabien Galthie era.
"It's amazing. We worked the whole week for a big game like that. It was a very tough game," he said.
"We have no experience, but we play with a smile. It was a good experience to win at the Principality.
"The try was a good moment. We don't think about the Grand Slam, we play match after match."
Captain Alun Wyn Jones said Wales only had itself to blame.
"The scoreboard said they deserved it," he said. "You make your own luck sometimes and we fell short on the scoreboard.
"We gave them two penalties and they scored. In the second half, we had the ascendancy and we felt we should have got more out of that scrum. There was a case for a couple of penalty tries.
"They have a very good defence coach [former Wales defence guru Shaun Edwards] and were an embodiment of that today."