After a cagey start, the encounter sprung to momentum with Thamsanqa Mkhize providing the first offensive movement out wide.
Sage Stephens got out of jail later on when he stopped Jeremy Brockie in his tracks from registering his first goal in Downs colours.
Approaching the 20th minute spell, the Citizens threatened to go in front as they found pockets of spaces in and around the visitors rear-guard.
Youngster Craig Martin broke free from a corner kick to set-up Lyle Lakay who forced Denis Onyango to make the game’s first goal-bound save after 22 minutes.
Referee Victor Gomes controversially sent off Sundowns coach Pitso Mosimane after the latter remonstrated a foul awarded against Tebogo Langerman three minutes later.
The game swung in both ends towards the interval, with the Brazilians slowly getting their mix, while Teko Modise looked to put one over his former club with a free-kick on the edge of the box.
Downs rang the changes after the break, bringing in Themba Zwane for an ineffective Sibusiso Vilakazi. Khama Billiat separated matters with a sublime volley following a clever assist by Percy Tau on the touchline.
In an effort to change the scoreboard, City turned to their bench and brought on Ayanda Patosi for Thabo Nodada.
The once Belgian-based player found a perfect opportunity to level proceedings minutes after his induction, but his attempt lacked the desired effect.
Mosimane’s side looked to close shop with a quarter of the contest remaining, by bringing off Brockie for defender Motjeka Madisha.
Aubrey Ngoma got a rosy homecoming after his introduction in the 77th minute, where he replaced the goal-scorer.
City continued searching for the equalizer, with Patosi from range minutes before the end but Onyango was well alerted to the danger.
They were reduced to 10-men after captain Robyn Johannes got his marching orders for a foul on Tau in added time.
The first ever victory over the Citizens stretch Downs lead at the top of the standings to seven points.