Brian Cody's Kilkenny led by just a goal, 1-12 to 0-12, at half-time but it was all too easy in the end.
Westmeath defended valiantly at times and had an excellent marksman in Killian Doyle who finished with 0-14, 11 coming from frees.
But they were overpowered by the visitors in the second period, with the introduction of TJ Reid at half-time adding bite to the Kilkenny attack.
The Cats’ goals in Mullingar came from Mikey Carey, Billy Ryan, James Maher, Walter Walsh and Reid.
Westmeath got a consolation goal from sub Owen McCabe in added time.
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