Ryan Giggs scored the second-half winner for Manchester United as they saw off a spirited Watford challenge.
Giggs made Watford pay for missing a golden chance
Giggs, making his 600th United start, pounced on a Matt Spring mistake and rounded keeper Richard Lee before finishing high into the net.
United had taken the lead on 12 minutes when Louis Saha set up Mikael Silvestre to drive low into the corner.
But Watford did not roll over and Damien Francis levelled before half-time from Ashley Young's cross.
It could have been even better for the Hornets had they taken a gilt-edged chance at the start of the second half.
Darius Henderson stabbed Marlon King's deflected free-kick across goal but neither Francis nor Danny Shittu could finish and Giggs made them pay shortly afterwards as United kept up their 100% start to the season.
It meant victory for the oldest manager in the Premiership over the top-flight's youngest, but Hornets boss Aidy Boothroyd should be encouraged by his side's battling display.
United arrived at Vicarage Road in sizzling form, having scored eight goals in their first two games, and it did not take them long to add to that tally.
Cristiano Ronaldo, booed once again by the opposition fans, might have breached the Watford defence even earlier but saw his header fizz narrowly wide.
Two minutes later, a crisp passing move ended with Saha providing the perfect ball for the overlapping Silvestre to power a low shot into the far corner.
Conceding a goal after just 12 minutes did not bode well for the newly-promoted side and they did well to keep the scoreline down as United went for the kill.
In an intense spell of pressure, stand-in keeper Lee, playing instead of the ineligible Ben Foster, somehow kept out Saha's flicked header.
Watford were under the cosh and Boothroyd sensibly responded by introducing midfielder Spring in place of winger Hameur Bouazza.
It had the desired effect and the hosts drew level within four minutes.
Vicarage Road erupted as Young tricked his way past Silvestre on the right and crossed for Francis to tap in at the far post - his second goal of the season.
The unlikely scoreline made for a jubilant atmosphere among the home fans at half-time but it did not last long as the hosts quickly learned one of the key lessons of Premiership survival: take your chances.
Henderson could have scored from King's free-kick but stabbed the ball across goal instead where both Shittu and Francis stood and watched as it rolled to safety.
Moments later, United were back in front.
Spring wanted too long on the ball and under pressure from Darren Fletcher, he gave it to skipper Giggs, who sidestepped Lee and swept home.
Watford fought hard to find an equaliser, but they found clear-cut chances hard to come by.
Saha should have wrapped it up at the death when he burst through the middle only to shoot straight at Lee, but unlike Watford's earlier miss, it did not prove costly.
Watford boss Aidy Boothroyd:
"We didn't play anywhere near well enough today. I know all my players can and will give so much more.
"I listen to managers saying 'I can't ask for more' but I do ask for more and I want more and I'll get more because they're fantastic players.
"Being average and losing to Manchester United doesn't interest me."
Manchester United boss Sir Alex Ferguson:
"Watford make it a real battle for you, but when we had chances to counter-attack, we should have kept the ball better.
"It really needed someone to put their foot on the ball and enjoy passing the ball, but I don't think they did that.
"But you saw another side to us today. We had to battle to win the points."
Watford: Lee, Doyley, DeMerit, Shittu, Powell, Young, Francis, Mahon (Bangura 75), Bouazza (Spring 31), King, Henderson (Stewart 65).
Subs Not Used: Chamberlain, Mackay.
Goals: Francis 34.
Man Utd: Van der Sar, Brown, Silvestre, Ferdinand, O'Shea, Ronaldo, Fletcher, Carrick (Richardson 75), Giggs, Saha, Solskjaer (Park 59).
Subs Not Used: Kuszczak, Evra, Rossi.
Goals: Silvestre 12, Giggs 52.
Ref: P Dowd (Staffordshire).