Sundowns had to remain patient after a goalless first half. They eventually found their breakthrough through Kutlwano Letlhaku shortly after the interval.
The result takes the African champions six points clear at the summit.
Siwelele FC remain without a win after another difficult night for the managerless side.
MAMELODI SUNDOWNS BREAK SIWELELE RESISTANCE
Siwelele frustrated the hosts during the opening 45 minutes as caretaker coach Godfrey Mosoetsa set up his team to defend in numbers.
Sundowns controlled much of the possession and territory but struggled to find a way through a disciplined Siwelele defence.
Goalkeeper Tshigwana Gift Mthombeni also produced a solid performance between the posts as the visitors managed to keep the reigning champions at bay.
The two teams went into the halftime break locked at 0-0, with Khulumani Ndamane picking up the only booking for Sundowns in the opening period in Tshwane.
Cardoso eventually turned to his bench for fresh energy and the decision paid off.
In the 58th minute, Kutlwano Letlhaku broke the deadlock for Sundowns, with the goal confirmed after a VAR check.
The goal changed the complexion of the match as Sundowns gained greater control and forced Siwelele FC to chase the game.
Mosoetsa responded with changes of his own, introducing Hamza Issah and Thato Letshedi, but the visitors could not find a way past Ronwen Williams.
SIYANDA NDLOVU INSPIRES LATE ROUT
Sundowns eventually made their superior squad depth count in the closing stages.
Siyanda Ndlovu, wearing number 38 after coming off the bench, doubled the advantage in the 79th minute with a superb finish to put Sundowns firmly in control.
Siwelele were unable to respond as the game entered stoppage time, and Ndlovu then produced another decisive moment.
The Sundowns substitute turned provider with an excellent through ball for Tashreeq Matthews, who showed composure to produce a clinical lob over the goalkeeper and make it 3-0.
Also read: Transfer report: Sundowns beat Pirates to sign Bafana star Matuludi
The final whistle confirmed another important victory for Sundowns, who continue to set the pace at the top of the Betway Premiership.
For Siwelele, however, the difficult start to the campaign continues, with the club still searching for its first victory.
