The Swedish striker guided home a first-time finish following Jacob Murphy's knockdown eight minutes into the second half, to put the Magpies 2-0 ahead, before ultimately winning 2-1 at Wembley.
It is a first major trophy for Newcastle since an FA Cup triumph in 1955 and the 25-year-old understood how much it means to the club.
"It's fantastic. The club has been waiting for this for 70 years and it feels completely incredible," he told Viaplay after the match.
"It means everything. I've been in this club for a few years now, created relationships, and I think these players deserve this. And the supporters, they've been fantastic."
It could have been more for Isak - two minutes before his goal, he pounced on a rebound as Liverpool failed to clear a corner.
But his strike was ruled out as Bruno Guimaraes was in an offside position and stood in Reds' goalkeeper Caoimhin Kelleher's line of sight when the initial shot came in.
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