Ayoze Perez headed Newcastle to their first Premier League win of the season at the 11th attempt as Watford were made to pay for missed opportunities.
Perez glanced fellow substitute Ki Sung-yueng's driven 65th-minute free-kick home to claim a priceless 1-0 victory at the end of another testing week for Rafael Benitez and his players.
Benitez had backed skipper Jamaal Lascelles' call for a united front in the run-up to the game, and he certainly got it as a team which lost Yoshinori Muto, Lascelles and Jonjo Shelvey to injury inside six minutes either side of half-time scrapped its way to the finishing line in front of a vociferous crowd of 49,157.
But the Hornets left Tyneside wondering just how they were doing so empty-handed after Gerard Deulofeu and substitute Stefan Okaka spurned gilt-edged opportunities and Roberto Pereyra rattled the crossbar.
Watford, who won 3-0 at St James' Park last season, showed early glimpses with Pereyra prominent, but so too did the Magpies, for whom Kenedy fired wide from distance after escaping the attentions of Abdoulaye Doucoure and Etienne Capoue.
However, the visitors should have been ahead with 15 minutes gone when defender Adrian Mariappa was left unmarked at a corner and saw his initial header blocked by Lascelles before shooting wide after Will Hughes' follow-up had been repelled by Mohamed Diame.
Keeper Ben Foster had to beat away Diame's fiercely-struck 20th-minute shot as Newcastle responded, although it took a fine intervention by DeAndre Yedlin to deny Deulofeu after he had broken clear seconds later.
Deulofeu side-footed wastefully wide from Jose Holebas' cut-back, but was even more profligate with 27 minutes gone when the visitors broke from a Shelvey corner.
Striker Isaac Success raced clear before picking out the unmarked Spaniard, only to see him steer the ball past the post as keeper Martin Dubravka advanced.
Capoue and Shelvey both curled free-kicks over and Deulofeu fired into the side-netting after being played into space by Success as half-time approached, but the game remained goalless as the whistle sounded.
Watford re-emerged in determined mood and Dubravka was a relieved man to see Pereyra's curling 50th-minute effort come back off the crossbar after Success had muscled his way past Yedlin and picked him out.
The Magpies' fortunes took a turn for the worse when Shelvey limped off seconds later, although they were spared further punishment when defender Craig Cathcart headed over from a Holebas corner.
But as the half wore on, it was the hosts who started to exert their authority, almost for the first time, and they got their reward with 25 minutes remaining when Perez got his head to Ki's driven free-kick and glanced the ball past the helpless Foster.
Rondon headed wide from Kenedy's cross four minutes later as the Magpies looked to kill the game off, but they were almost caught after very nearly doubling their tally amid a 73rd-minute goalmouth scramble when Watford raced up-field, only for Kenedy to make a superb challenge to deny substitute Andre Gray.
Okaka should have levelled deep into stoppage time but hooked over from close range.