After Fiorentina - who United beat 2-0 on Saturday - overcame Nice in the day's opening game, the Magpies knew that they would be able to claim the trophy in front of their own supporters if they avoided defeat against their La Liga opponents.
In the event, a brace from Harvey Barnes on his home debut and goals from Jacob Murphy and Joelinton gave them a thoroughly deserved victory against El Submarino Amarillo, who finished fifth in Spain's top flight last season.
Eddie Howe named a completely changed starting XI to the one which overcame Fiorentina the previous day, with summer signing Barnes playing at St. James' Park for the first time as a Newcastle player and Sean Longstaff returning from injury.
Wearing their new carbon and navy blue third kit, they made an electric start and took the lead in the sixth minute. Matt Ritche's delivery from the left was only half cleared and Murphy took a couple of touches before lashing a shot from just inside the area beyond Pepe Reina.
Four minutes later, United had a goal chalked off when Ritchie's free kick was played short to Bruno Guimarães to whip in a dangerous delivery, and Callum Wilson diverted the ball home - only for the flag to go up for offside.
Then in the 28th minute Newcastle had another goal disallowed. From another Ritchie set-piece, Reina was unable to collect and Paul Dummett forced the ball over the line but was adjudged by referee Anthony Taylor to have fouled the 40-year-old former Liverpool and Barcelona goalkeeper, now in his second spell with Villarreal.
Reina then denied Ritchie, palming away the long-serving wide man's shot after good approach play from Barnes and Wilson.
At the beginning of the second period, Ramon Terrats fired over for the visitors before Barnes cut inside for United, driving an effort just wide of the far post at the Gallowgate End.
Two minutes later, the former Leicester man tried his luck again, this time after being slipped in by Guimarães, but Reina blocked his effort.
Alexander Sorloth, the ex-Crystal Palace forward, curled wide after dispossessing Jamaal Lascelles then Carlo Adriano was thwarted by Nick Pope as Quique Setién's side threatened to equalise.
But Barnes got the goal his performance warranted on his home bow just after the hour mark. Wilson found Joelinton, who unselfishly played in Barnes to slot home. The winger celebrated by the Strawberry Corner and was serenaded with a chant of "super Harvey Barnes," having made an immediate impact in front of his new supporters.
Pope comfortably fielded a snapshot from Villarreal captain Gerard Moreno but the Magpies made it 3-0 in the 75th minute when Joelinton was allowed to run from just inside the opposition half and took full advantage, his low shot from edge of area kissing a post on the way in.
And three minutes later, it was four for the Magpies and two for Barnes. Wilson burst past Abraham del Moral on the right and cut the ball back for Barnes, who drilled home.
Moreno scuffed an effort wide for the visitors in the 88th minute and youngster Amadou Diallo came on for his senior debut in the closing stages as United made it two wins in two days against high-calibre European opposition, without conceding a goal, and were able to celebrate silverware at the end of a superb festival of football, which was watched by a total attendance of 71,346 across the weekend.
