The Warriors were good value for their six-point victory, with the bonus point won by both sides leaveing Ulster three points ahead of the Scots with six rounds of matches remaining.
With his squad severely depleted by injuries and international call-ups, Ulster head coach Mark Anscombe made wholesale changes to the province's starting line-up.
Winger Neil Walsh and young back rower Conor Joyce made their full debuts and among the backs, both Andrew Trimble and Paddy Wallace lined up in the unfamiliar roles of outside centre and out-half respectively.
The RaboDirect PRO12 leaders also fielded a new-look pack which saw Callum Black, Nigel Brady, Neil McComb and Ali Birch all making rare starts.
The Warriors escaped unscathed from an early Pienaar penalty attempt after collapsing a second minute scrum, but the Springbok made no mistake in the sixth minute, nailing a central kick from distance after Tongan prop Ofa Fainga'anaku was whistled up for killing the ball.
Glasgow responded well with sustained pressure from the restart, but failed to capitalise when Horne's 12th-minute penalty attempt fell short of the posts.
The hosts continued to press, however, and were well worth the 15th minute overlap try touched down by second row Tim Swinson from Mark Bennett's pass.
Horne's conversion effort veered wide, however, which enabled Ulster to edge back into the lead three minutes later as Pienaar fired over from close range after Josh Strauss had illegally impeded Birch off the ball just metres from the try-line.
The teams roughly shared possession and territory as the first half progressed, but a mix-up at an Ulster scrum on 28 minutes allowed the Scots strike again.
Pienaar seemed to take his eye off the ball for long enough to allow his opposite number Nikola Matawalu in to pilfer and Tommy Seymour scooted in at the corner for his second try of the season against his former province.
Horne's miserable night from the boot continued though as his conversion again spun wide, and the score stood at 10-6 to the hosts.
Ulster offered up a spirited reaction, with several phases of rapid attacking eventually forcing the Glasgow defence into another infringement on the 22. Pienaar's kick uncharacteristically spiralled wide of the left hand upright to ensure the visitors ran off four points adrift at the break.
The second period got off to a nightmarish start for Ulster. Peter Murchie charged down Wallace's clearance attempt metres from the try-line and was quickest to react as the ball dropped for the simplest of tries.
Horne missed his conversion effort once more but with the Warriors now overflowing with confidence, it took assiduous defending from every Ulster player to resist the blue tide as it surged forward once again straight from the restart.
Pienaar closed the gap to six points courtesy of a 49th minute penalty to keep his side in contention. With Darren Cave, Paul Marshall and Rob Herring all entering the fray, Ulster then enjoyed their best period of possession in the entire match.
Frustratingly, however, they failed to register any further points during this 10-minute purple patch which ended with the overruling of a Robbie Diack touchdown by referee Nigel Owens for a very marginal forward pass.
With 67 minutes on the clock, Ulster's dominance finally translated into points with a well-executed overlap on the wing which saw Cave find Stuart Olding wide on the right for the youngster's first senior try.
Pienaar's acutely-angled conversion missed the posts, leaving the Scots 15-14 ahead with less than a quarter of an hour remaining.
Man-of-the-match Matawalu proved to be the difference on 73 minutes, finishing off a move he himself had started with a flicked sideways pass to Graeme Morrison.
The in-form Fijian then succeeded in outpacing the Ulster defence as he retrieved the ball to ground it behind the posts.
Inexplicably, Horne's elementary conversion somehow screwed wide of the uprights, giving Ulster a brief glimmer of hope as a converted try would have given them the verdict by a solitary point.
The visitors gave it their all as the clock wound down, but as Walsh ran out of space on the right wing with only 60 seconds remaining their faint chance evaporated into the chilly Glaswegian air.
The only consolation for Anscombe's side was the losing bonus point which ensures that they head into round 17 with a three-point lead over the Warriors at the top of the table.
|Glasgow Warriors Score Card|
|Ulster Rugby Score Card|