Paddy McNair’s excellent first-half free-kick decided the game as Boro produced an improved performance following a surprising defeat at York City earlier in the week.
Trialists Sol Bamba and Jamie Paterson also came off the bench in the second half as both sides made plenty of changes to give players minutes ahead of the new campaign.
Boro will play their Championship opener at Fulham in just over a week’s time on Sunday, August 8.
Warnock will hope he can make more signings before the game at Craven Cottage and has admitted his squad remains thin following several first-team departures this summer.
Marcus Tavernier, Marc Bola and Sammy Ameobi all missed the Rotherham game through injury.
Here’s how each Boro player fared.
1. Joe Lumley - 7
Didn’t have much to do but made a fine save to keep out Dan Barlaser’s free-kick in the second half. 7
Photo: Getty Images
2. Anfernee Dijksteel - 7
Still a little hesitant to get forward in the right wing-back role. Was able to make a few more forward runs in the second half. 7
Photo: George Wood
3. Paddy McNair - 9
Excellent free-kick decided the game. Looks suited to his role on the right of a back three where he is composed on the ball, can step out of defence and deliver crosses. 9
Photo: Stu Forster
4. Grant Hall - 7
Solid at the back and won the majority of his headers. Kept Rotherham frontman Michael Smith quiet. 7
Photo: Stu Forster