Jackie McNamara's side celebrated victory in the Tayside derby after racing into a 3-0 first-half lead against Tommy Wright's lacklustre visitors.
Goals from Keith Watson, David Goodwillie and Gary Mackay-Steven before the break put the outcome beyond any doubt before midfielder Stuart Armstrong completed the rout with their fourth after 52 minutes.
United were without suspended defender Gavin Gunning but new signing Mark Wilson was on the bench.
Former United keeper Steve Banks was back at Tannadice for the first time since leaving in the summer as he kept his place in the absence of injured Northern Ireland ace Alan Mannus.
But within just four minutes Banks was beaten as the home side broke the deadlock.
Ryan Dow's pass found Watson and the full-back had only one thing on his mind as he advanced 22-yards from goal.
Watson took a touch before unleashing a powerful left-foot drive which took a deflection off Patrick Cregg to beat Banks into his far corner.
It was a perfect start for the hosts, who were intent on making their home patch something of a fortress again.
But they were almost punished after 10 minutes when Cregg's cute cross was headed over the bar from six yards by Peterborough-target Stevie May.
However, after 25 minutes, Goodwillie made it 2-0 for the hosts.
Dow's free-kick deep into the visitors penalty area found Watson, who headed it towards Goodwillie and the on-loan Blackburn striker rose in style to nod past a helpless Banks.
The home team were buzzing now and almost went further ahead on 28 minutes but Banks defied his old club by pulling off a super save from Goodwillie, whose shot was deflected off Tom Scobbie only for the veteran Saints keeper to tip it wide.
After 40 minutes, though, Mackay-Steven made it 3-0.
Calum Butcher released full-back Andrew Robertson, who ran down the left before slipping in Mackay-Steven, and the Arabs winger calmly rolled his finish under Banks, who had raced off his line to delight the home fans going into the interval.
Chris Millar replaced Cregg at the start of the second half for Saints in a bid to signal some sort of revival.
Nigel Hasselbaink had a chance within minutes of the restart when he latched on to a loose ball but shot straight at home keeper Radoslaw Cierzniak from 10 yards.
But after 52 minutes, United killed the contest with a well-worked goal.
Dow slid the ball wide to Mackay-Steven, who in turn crossed for Armstrong.
Armstrong timed his run to perfection from midfield and although his shot hit the ground, it bounced up to loop over stranded Banks.
Saints striker Steven MacLean hooked a shot over from point-blank range after 60 minutes, but United remained in complete control towards the end to secure the win which could finally kick-start their season.