Warne, who grew up supporting the Canaries and had his family in the stands, saw his side go in front through Richie Towell and a shock against the high-flying hosts was on the cards until a second-half turnaround at Carrow Road.
Todd Cantwell, Max Aarons and Teemu Pukki struck after the break to end the Millers' seven-game unbeaten run and go back to the top of the Championship.
Ending the Millers' lengthy wait for an away win at this level at Norwich could not have been more fitting for Warne and it looked a possibility as his side surged into an 11th-minute lead.
Towell poked home from close range after Tim Krul could only parry Will Vaulks' stinging effort from the edge of the box.
They were almost in dreamland nine minutes later but Ryan Williams struck the inside of the post with a good effort.
The second half was a different story and the Millers could not cope with Norwich's pressure, though they did not help themselves.
A spell of home pressure was eventually rewarded in the 55th minute as Cantwell struck his first senior goal with a sweet finish from close range after team move.
The lead came 16 minutes later as Aarons also notched his first goal, nodding home from Cantwell's clever cross.
Pukki missed a sitter to make the game safe, but he made up for it after some shoddy defending from the Millers, as Marco Stiepermann robbed Semi Ajayi of possession and fed Pukki to slide home.
It confirmed a first defeat since the start of October for the Millers, who remain three points off the relegation zone.
Norwich: Krul, Arrons, Zimmermann, Klose, Lewis (Rhodes 87), Tettey, Vrancic (Trybull 89), Emi, Stiepermann, Cantwell, Pukki (Godfrey 90)
Subs not used: McGovern, Marshall, Passlack, Srbeny
Rotherham: Rodak, Vyner, Ajayi, Robertson (Wood 46), Mattock, Vaulks, Williams, Towell, Newell (Taylor 42), Forde (Proctor 73), Smith
Subs not used: Price, Manning, Wiles, Jones
Referee: Stephen Martin