The Millers took the lead after 10 minutes when, after seeing his first effort blocked by Gulls goalkeeper Scott Bevan, Ryan Taylor poked home.
Torquay equalised after 31 minutes when Chris Zebroski raced to the edge of the area and his angled shot across Jamie Annerson’s goal went in off the post.
But Rotherham’s response was swift. Nicky Law’s 33rd-minute corner looked to have been cleared, but Danny Harrison drilled the ball through a crowded goalmouth and into the net.
And they should have had a third just before the break when Ryan Cresswell had a clear chance from Harrison’s corner, but he directed his header over the bar.
The Millers finally wrapped up a first win under Andy Scott 10 minutes from time with substitute Oliver Banks marking his senior debut by firing home from Adam Le Fondre’s pass.