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.
5. Dael Fry - 8
Posed a threat in the air from set-pieces and was assured at the back. 8 Photo: Getty Images
6. Isaiah Jones - 5
Started in the left wing-back role but was unfortunately forced off on 27 minutes after a blow to the nose. 5 Photo: JPI Media
7. Sam Morsy - 8
Industrious in midfield, preventing Rotherham attacks and winning loose balls. Looks like he’ll be a key player this season. 8 Photo: Catherine Ivill
8. Jonny Howson - 6
A few heavy touches in the first half. Still getting up to speed after missing most of pre-season. Offers defensive protection alongside Morsy in midfield. 6 Photo: Stu Forster