Dwight Gayle put United ahead at Vicarage Road, making no mistake from just a few yards out to give the visitors a lead they held until the break.
But when Kiko Femenia went down under Matt Ritchie’s challenge on 52 minutes, referee Craig Pawson pointed to the spot and Deeney dispatched it.
And with just ten minutes remaining, another penalty decided it when Javier Manquillo was penalised for a foul on Ismaila Sarr and Deeney again stepped up to score – boosting the Hornets’ survival hopes in the process.
Deeney’s brace confirmed just a second defeat in eight Premier League games for Steve Bruce’s men, who will now get set to welcome Tottenham Hotspur to St. James’ Park on Wednesday night.
United started well and could have gone ahead inside ten minutes. Deeney got a touch to Jonjo Shelvey’s inswinging corner from the left but the ball fell kindly for Miguel Almirón at the far post. His half volley looked destined for the net, but Ben Foster got across just in time to parry it away.
The hosts went ever closer minutes later. Danny Welbeck found space in the box and sent a first time shot goalwards which had Martin Dúbravka struggling to reach it, but as it bounced off the inside of the post, Jamaal Lascelles – back in the side after injury forced him out of Wednesday night’s defeat to Manchester City – was there to clear it just before it crossed the line.
Midway through the half, though, Bruce’s men went ahead. Ritchie’s corner was glanced across the face of goal by Federico Fernández and Gayle was perfectly placed to prod home the opener from inside the six-yard box.
It was a much-improved first half showing after the heavy midweek loss at the Etihad Stadium, and Newcastle could have moved further in front before the break. Gayle’s clever lay-off, at full stretch, sent Saint-Maximin clear with just Foster to beat. The Frenchman took aim with the outside of his right foot, but just couldn’t quite lift his shot over the Hornets’ stopper’s outstretched leg.
Gayle hooked a Ritchie cross over the bar just after the restart but seven minutes in, Nigel Pearson’s men were handed the opportunity to level the scores from the spot. Femenia’s run took him into the box and as he nudged the ball past Ritchie, the slightest of touches sent him tumbling and referee Craig Pawson pointed to the spot. Deeney’s penalty flew straight down the middle, evading Dúbravka’s dive.
Deeney drew a save from Newcastle’s Slovakian stopper and then saw a shot deflected over by Danny Rose as Watford rallied, but United had the ball in the net with a quarter of the contest remaining. Saint-Maximin broke away and his square ball was inadvertently turned in by Craig Dawson. VAR checked the incident, but it was apparent that the Magpies flier was offside at the start of the move.
But with ten minutes to go, another spot kick was awarded, and it proved decisive. Manquillo held on to Sarr for too long as he turned into the box and Pawson pointed to the spot. Deeney steadied himself and fired it down the middle once more, with Dúbravka’s dive again beaten and the Magpies unable to muster a response in the closing stages.
