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.
9. Matt Crooks - 6
Dropped into midfield at times and played a few neat passes. Found it harder to get on the ball in more advanced positions. 6 Photo: Getty Images
10. Duncan Watmore - 6
Made some probing runs off the ball yet the forward was on the fringes of the game. Subbed at half-time. 6 Photo: George Wood
11. Josh Coburn - 7
Won the free-kick for McNair’s opener and put himself about against the three Rotherham centre-backs. Almost added a second before he was subbed at half-time. 7 Photo: Stu Forster
12. Jack Robinson - 6
(for Jones, 27) - Made a few loose passes after coming on at left wing-back but continued to provide an outlet and tried to get on the ball. 6 Photo: Pool