Namibia international Heinrich Isaacks fired the Team of Choice ahead in the first half but they were unable to hold on for maximum points as they conceded a goal in the dying seconds of the game.
The hosts fired the early warning shots with Ashley Hartog testing Thela Ngobeni with a low shot in the ninth minute while Bongolwethu Jayiya did the same two minutes later.
Sundowns - who were chasing an 11th straight win in the League - then started to get into the game with Hlompho Kekana firing wide in the 12th minute, while Lebohang Mokoena also missed the target in the 25th minute.
The opening goal arrived in the 34th minute as Isaacks sidefooted home at the far post, connecting with a pinpoint low cross from Kurt Lentjies.
United almost extended their advantage in the 49th minute when Jayiya received a cross from Lentjies on the left side of the box, but he sent his shot over the target.
Two minutes later United's Virgil Vries made his first real save of the game, using both hands to punch a well-struck shot from Anthony Laffor to safety.
The chances were few and far between in the second half but Downs defender Thabo Nthethe came close in the 66th minute, heading over the crossbar from Elias Pelembe’s corner.
Jayiya had an opportunity to seal the win for the home side four minutes into added time when he used his skill to create space inside the box, but his shot flew well over the crossbar.
Sundowns drew level a minute later as their captain Schut rose highest at the near post to direct a powerful header past Vries and into the back of the net.