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.
13. Sol Bamba - 7
(for Hall, 45) - Didn't shy away from challenges and showed his experience with a strong display at the back. 7 Photo: Getty Images
14. Uche Ikpeazu - 7
(for Watmore, 45) Another impressive display from the frontman who gave Boro more of a focal point in the second half. Held the ball up well and brought others into play. 7 Photo: Catherine Ivill
15. Chuba Akpom - 5
(for Watmore, 45) Played alongside Ikpeazu but struggled to cause much of a threat. 5 Photo: Jacques Feeney
16. Nathan Wood - N/A
(for Fry, 70) Didn’t have much defensive work to do in the closing stages. N/A Photo: Stu Forster