Nyasha Mushekwi got one back for the Brazilians on 90 minutes, but it was too little too late.
Maritzburg had a chance to grab the fastest goal of the season when straight from the kick-off a right-footed shot by Cuthbert Malajila from 35 metres out missed to the left.
The Brazilians had an early chance in the fifth minute, but Edward Manqele did not connect properly with a right-footed shot from the right side of the box, and his effort was saved by Shu-Aib Walters.
In the 30th minute Sundowns initiated an enterprising move which thrilled the fans when Hlompho Kekana played a one-two with Lebohang Mokoena, but his shot went wide and high.
Four minutes later Sundowns thought that they had found the opening goal from a corner kick, when Wayne Arendse headed towards goal, but the ball was headed off the line by Peter Petersen.
The visitors continued attacking and a minute before half-time Malajila had a shot saved by Wayne Sandilands' feet, when it had looked like he was beaten.
Maritzburg coach Ernst Middendorp was pleased with his side's first half performance and was urging his troops to continue and press had in the second.
And United did take the lead in the 70th minute when Buckley unleashed a trademark left-footed shot to make it 1-0.
Four minutes later the former Bafana Bafana winger made it 2-0 in when he linked up with up with Malajila and unleashed a shot from close range that Sandilands could not handle.
Sundowns showed some late fight when they grabbed a goal, but it was too late to mount a fight-back. Teko Modise linked up with Mushekwi and the Zimbabwean international scored with a well taken shot.
As the fulltime whistle rang there were ugly scenes at the stadium when Sundowns supporters rioted with security guards, baying for coach Johan Neeskens' blood.