Mor Diouf, Diyo Sibisi and Rudolf Bester got the goals for Maritzburg, before Eduardo Ferreira pulled one back for The Brazilians.
With the result Downs' title hopes are extinguished and United's place in the top-flight is safe, as Vasco da Gama lost in their League match.
It was a great start for the home side as they found the back of the net in the fifth minute after some poor goalkeeping by Wayne Sandilands.
United whipped in an in-swinging free-kick from the left side of the field and the Downs keeper spilled the cross straight into the path of Diouf six yards.
The Maritzburg player made no mistake, side-footing the ball into an empty net.
The Brazilians seemed a little shocked after a blistering start by the Team of Choice and were lucky not to go a second goal downs eight minutes later.
Gert Schalkwyk found some space just inside the 18-yard area after a great passing movement, but he squandered the golden opportunity when his shot sailed just past the upright.
Sundowns started to find their footing and hit the woodwork in the 15th minute.
Katlego Mphela struck a powerful 30-yard free-kick goalwards, but his effort deflected off the defensive wall and completely wrong footed Shu-Aib Walters.
Luckily for Walters the ball struck the right-hand post.
After that, however, it was Maritzburg who played the better football as they stroked the ball around the park and opened the Downs defence with ease, but their finishing was lacking.
But when United did manage to get their shooting right Sandilands was there to deny them.
In the 28th minute the Downs keeper did brilliantly, diving to his right to push away a 20-yard Rudolph Bester shot.
Nonetheless, there was nothing Sandilands could do in 40th minute when Diyo Sibisi scored from the penalty spot.
The referee adjudged Ferreira to have fouled Sibisi inside the box, and the striker doubled his side’s lead, giving them a firm advantage going into the half-time break.
The second half continued in the same fashion and United put the match beyond doubt when they found their third goal in the 53rd minute.
Another swift passing movement by the Team of Choice, and Bester found himself in space 16 yards out and buried the ball with a firm strike to make it 3-0.
Downs pulled a goal back in the 62nd minute, when Ferreira made up for his earlier indiscretion, pouncing on a loose ball in a goalmouth scramble and firing it into the back of the net.
Thereafter The Brazilians pressed forward in the hope of an amazing comeback, but it was not to be.