After a goalless first half Ross Stewart scored twice after the interval following two crosses from Jack Clarke.
Clarke then got on the scoresheet after an excellent run and finish, before he was substituted to a standing ovation.
The result means Sunderland have taken 11 points from their first seven league games this season.
Our chief Sunderland writer Phil Smith has dished out his player ratings from the Stadium of Light:
1. Anthony Patterson - 6
Another game where he conceded with little hope of doing much about it. Not really tested in the end, solid in terms of his distribution for the main. 6 Photo: Frank Reid
2. Trai Hume - 6
One of many who was a touch off the pace in the first half but looked so much more like his old self in the second. Crashed into challenges, broke into dangerous areas, important in the win. 6 Photo: Frank Reid
3. Dan Ballard - 7
Some loose passes in the first half in particular but is having a good start to the season and was so powerful in getting to the ball at times, snuffing out some threatening attacks. 7 Photo: Frank Reid
4. Luke O’Nien - 6
There was frustration when Rotherham scored a soft goal but he showed why he’s in the role by playing a part in the equaliser by playing out from the back shortly after. Defensively had a very good second half, and was brave in possession throughout. 6 Photo: Frank Reid