Tony Mowbray made a wining start to life as Sunderland boss as his new charges ended Rotherham's unbeaten start to the season.
Ross Stewart scored his first with a cool finish from Jack Clarke's low cross and the pair then linked up again for Stewart's second.
The impressive Clarke then rounded off the win with a wonderful solo goal, as he weaved past a couple of Millers defenders and then fired a low shot into the bottom corner.
A first home victory of the season for Sunderland saw them move up to eighth in the table, three points behind early leaders Sheffield United.