Diack's first-half score and Henderson's effort after the break were supplemented by some superb goal-kicking from man-of-the-match Ruan Pienaar.
For Leinster it was once again the Ian Madigan show, the 24-year-old fly-half kicking six penalties from six but his side failed to get over the whitewash, with some last-ditch defending from Ulster denying them at the death.
The win was greeted by near total silence at a stunned RDS but comes at the perfect time for an Ulster side that had struggled to match their whirlwind start to the season in recent weeks.
The last time Leinster suffered defeat in the RaboDirect PRO12 was back in December, when Ulster prevailed 27-19 at Ravenhill and Mark Anscombe's side came roaring out of the blocks this time around as well, a well-rehearsed backs move putting Andrew Trimble through a hole in the Leinster midfield early on.
But after phase after phase of play on the Leinster tryline it was the returning Springbok scrum-half Pienaar who knocked on and the chance was gone.
And after a quarter of an hour Madigan, Leinster's hero against Glasgow last weekend, had notched the first points of game, slotting a penalty to put the hosts 3-0 up.
Ulster were the dominant force in the first half despite Madigan slotting over another penalty from an almost identical position to the first only moments later.
But in the 22nd minute the visitors had the first try of the match, and deservedly so. No. 8 Nick Williams busting a hole up the middle before Paddy Jackson spread the ball left to Diack who muscled over in the corner.
Jackson missed with the conversion to leave the score at 6-5 to Leinster but no sooner had Ulster closed the game than the hosts pulled away again, winning a penalty at the scrum that Madigan duly slotted away.
Anscombe's men were very much on the front foot still as the half-time whistle approached, full-back Jared Payne knocking on when well-placed on the right but Pienaar's first penalty of the night made it 9-8.
That looked like being the end of the scoring for the first half but as time expired Madigan kept his cool to add his fourth penalty.
The kicking battle continued after the interval as two Pienaar penalties to one Madigan effort tightened things up again at 15-14 as the clock ticked round to the hour mark.
Craig Gilroy was a constant menace to the Leinster line and it was his break down the left that set up the next attack in the 64th minute.
The ball was spread back right and after Trimble was tackled just short of the line, Pienaar's quick wittedness allowed him to feed Henderson who plunged over in the right corner for their second try.
Pienaar missed with the tough conversion but at 19-15 Ulster were deservedly in front at the RDS and Leinster had it all to do.
But not for nothing are Joe Schmidt's side the champions of Europe and with the unerring Madigan in their side points come easily, Ulster again penalised at the scrum and the stand-off kicking his sixth penalty from six attempts.
With only one point in it heading into the final ten minutes the next score was always going to be vital, and it was Pienaar who held his nerve to slot a tricky penalty from out wide on the right and make it 22-18 to the visitors.
Ulster were good value for their lead but the set-piece was a continual source of frustration, Rory Best and his front row cohorts once again penalised with five minutes to go.
Right in front of the posts and with Madigan still faultless from the tee, Leinster instead opted to kick for the corner but some resolute defending on their try-line saw Ulster fight off the initial threat.
Phase after phase on their own try line saw Ulster defend manfully as the game ticked round to 80 minutes and then further as the referee played advantage to Leinster over and over for continual offside.
Anscombe must have wondered what else his side had to do to win the game, Payne sent to the sin bin as the game entered the fifth minute of time added on but the 14 men got their reward when Leinster's final surge was held up over the line.
|Leinster Rugby Score Card|
|Ulster Rugby Score Card|