The result spelt the end of a nine-game unbeaten run for the 2016 African champions as they failed to hold on to their lead.
Liberia international striker Anthony Laffor had gotten them ahead in the 38th minute before Yaya Kone's goal from the spot and Samuel Mathias' second-half winner earned the Pride of Benue a glorious Champions League start.
Sundowns got the game started but were immediately put on the back foot by the hosts who sought to dominate from early on. However, a lapse in concentration at the other end saw Laffor beat the offside trap, but Olufemi Kayode got off his marks to save their blushes.
The visitors grew into the game, imposing their bold yet risky approach as they combined patiently from deep before launching passes into the attack, with the Liberia marksman making dangerous dashes behind the hosts’ rather uncoordinated defence.
It was one of such approach that got Pitso Mosimane's men a corner kick, where the resulting delivery saw defender Ricardo Nascimento’s powerful goal-bound header met by a stunning acrobatic save by Kayode to again keep the scoreline level.
Laffor shot wide off another well-picked long pass that once again caught Lobi’s defence flat-footed.
Lobi Stars fluffed a rare chance to take the lead before the half-hour mark after a pass caught out Sundown’s defence on the right side of the box, but right-back John Lazarus’ cross lacked the pace and quality to outsmart Mweene.
In the 38th minute, Sundowns deservedly took the lead off a trademark move as Leandro Sirino got to the end of a defence-splitting pass from deep before the no. 10 squared for an easy tap-in from Laffor.
Shortly before the interval, Solomon Ogbeide’s wards levelled from the spot. Ricardo fouled Sunday Adetunji, clear on goal, in the area and Kone stepped up to send Mweene the wrong way.
Mweene produced two stunning saves in the 49th minute to deny Adetunji. Firstly, the Zambia shot-stopper got a hand to a header from the towering marksman before thwarting a superbly executed volley.
Immediately after Lobi had a goal ruled out for offside, Mathias relaxed any eyebrow it raised by getting them ahead in the game for the first time with a crisp left-footed finish.
In the 56th minute, Ebube Duru almost pulled them away further but his fierce cross cum shot cannoned off the crossbar.
The second half saw an improved performance from Lobi Stars, with their leaky defence upping the ante as the Brazilians sought to find a leveller.
Meanwhile, lanky striker Adetunji led the charge in keeping the PSL champions' backline on their toes as the hosts got off to a victorious start in Group A.