Newcastle rued plenty of missed chances after having 20 shots on goal but The Hornets also didn't have any luck on their side as Joshua King had a late goal ruled out by VAR.
Watford climb up to 11th in the Premier League table on seven points, while Newcastle also move up a place to 17th with three points from six matches.
Story of the game
Steve Bruce surprised Newcastle fans by introducing Joe Willock into the starting line-up, after previously confirming in his pre-match press conference that the 22-year-old was set to miss the game. Therefore, The Magpies' only change from the 1-1 draw with Leeds United arrived in defence as skipper Jamaal Lascelles was ruled out with a thigh injury and replaced by Federico Fernandez.
History was made ahead of kick-off at Vicarage Road as Jarred Gillett became the first overseas referee to take charge of a Premier League fixture after the Australian enjoyed a fruitful nine-year spell officiating in the A-League. Newcastle are already familiar with Gillett as an official after the 34-year-old refereed their EFL Cup Second Round match against Blackburn Rovers in September 2020.
Watford had an early sight at goal just two minutes in as Tom Cleverley sent Emmanuel Dennis away down the left-hand side and the Nigerian forward advanced into the area before being denied by a double save from Karl Darlow.
It took only six minutes for Newcastle fans to chant "We want Brucie out", as the sold out away end made their voices heard.
After a wobbly start, Newcastle grew into the game and won a flurry of corners in quick succession as Matt Ritchie's delivery was met by Fernandez and Ben Foster had to flick the Argentine's header over the bar. Longstaff then went close just two minutes later with an effort which went wide of the upright.
Newcastle, with their fifth corner of the match after 23 minutes, made their pressure count as The Hornets half cleared the ball but it swiftly made it's way into the feet of Allan Saint-Maximin. The Frenchman found Longstaff 22 yards out, and the midfielder, curled a splendid strike beyond Foster, who could only push it further into the top corner, as the away fans erupted with chants of "He's one of our own".
Munoz's side were stunned but they still looked to pose a threat as Sarr's 25-yard strike was palmed away by Darlow and William Troost-Ekong saw his shot from a similar position sail behind the goal.
Newcastle charged forward and appealed for a penalty after Troost-Ekong slided in with a late challenge on Willock on the edge of the box but the foul was right on the line and VAR agreed with the referee's decision with the awarding of a free-kick. From the resulting set-piece, Ciaran Clark headed Ritchie's delivery narrowly over the bar.
On the stroke of half-time, The Magpies should have doubled their advantage as Craig Cathcart gifted the ball to Saint-Maximin, who was through on goal and as he tried to go round Foster, the goalkeeper got something on it, so he picked out Longstaff from 12 yards but he somehow failed to grab his second of the afternoon and put it over the bar.
Watford started the second half on top as Dennis tried to cause problems on the left when he cut inside but the visitors defence held firm and made solid blocks to frustrate the Nigerian.
Foster was still by far the busier out of the two keepers and Newcastle were desperately seeking a second goal but couldn't find a way past the experienced shot-stopper as he got down quickly to deny Miguel Almiron's drive and Joelinton's near post effort. Willock was also left bewildered when he burst through on goal but saw his effort blocked by Foster.
In typical fashion, The Magpies' lack of finishing came back to haunt them as Watford equalised when Rose's corner was flicked on by King into the back post and Sarr, from eight yards, made no mistake, to set up a grandstand finish at Vicarage Road.
With nine minutes to go, Newcastle squandered another golden opportunity as Ritchie and Clark combined but the latter failed to hit the target.
In extraordinary scenes, Watford thought they had snatched all three points when Sissoko's shot was parried by Darlow and as King pounced on the rebound, the Norwegian was adjudged to have been offside from the Frenchman's shot and VAR correctly made the right decision to give Newcastle a huge sigh of relief.
Deep into stoppage-time, Bruce's side should have won it as Jacob Murphy, who replaced Almiron, raced through one-on-one after Saint-Maximin's well-weighted through ball, but instead of putting his foot through it, he tried to chip Foster and the keeper ended up making a comfortable save as Newcastle's wait for a first Premier League win this season goes on.
Sarr to the rescue for Watford
The Senegalese winger was largely kept quiet for much of the game but he's The Hornets top goalscorer for a reason and he was in the right place at the right time to grab a precious headed equaliser. It's no surprise that he is Watford's star man, and he will be imperative to their chances of securing Premier League safety at the end of the season.
Newcastle shot themselves in the foot
Before Watford's equaliser, the contest should have been out of sight. Newcastle spurned some incredible chances, notably the one just before the break with Longstaff and of course the one right at the end with Murphy. Defensively, they were a lot more solid at the back but the composure in front of goal has cost them two extra points.
Since Callum Wilson joined the club, Newcastle have now only won two of their 15 league fixtures without their target man. How much would The Magpies have benefitted with their number nine up front.
Watford: Foster; Femenia, Troost-Ekong, Cathcart, Rose; Kucka, Sissoko; Sarr, Cleverley, Dennis; King. (4-2-3-1)
Newcastle United: Darlow; Manquillo, Fernandez, Clark, Ritchie; Willock, Hayden, S. Longstaff; Almiron, Joelinton, Saint-Maximin. (4-3-3)