F-T: Newcastle United 4 Rotherham United 0
The bottom-placed Millers arrived in Tyneside with just one away point all season and frustrated the Magpies with a tenacious display in the opening period.
But they fell behind just seconds before the interval when Daryl Murphy latched on to Deandre Yedlin's supply to turn and fire home, and the Magpies, heading back to the top of the table, controlled proceedings after that.
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Hide AdRotherham had had the best chance of the opening period eight minutes earlier when home goalkeeper Karl Darlow let the ball slip from his grasp but then produced a stunning save to keep out Jerry Yates' goalbound shot.
It was a double let-off for Newcastle because it looked as though Darlow brought down Anthony Forde in the area in the build-up to Yates' eight-yard effort.
Rafa Benitez's side ended the contest by the hour mark as Matt Ritchie was on hand to score after Richard O'Donnell had parried Yoan Gouffran's shot and then Ayoze Perez despatched Yedlin's cross.
Ritchie added a fourth on 77 minutes after Jonjo Shelvey's run and pass.
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Hide AdGoalkeeper O'Donnell had been handed an instant debut after his move from Bristol City earlier in the week, while Rotherham top scorer Danny Ward was on the bench after his back problem.
With Kirk Broadfoot ruled out with a back complaint of his own, Aimen Belaid came in to partner Richard Wood in the centre of defence.
O'Donnell did nothing wrong but the Magpies were simply too good for the Millers.
Newcastle United (4-4-1-1): Darlow; Yedlin (Sterry 80), Lascelles, Clark, Dummett; Ritchie, Hayden, Shelvey, Gouffran (Ameobi 64); Perez; Murphy (Lazaar 75) Subs not used: Sels, Haidara, Hanley, Barlaser.
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Hide AdRotherham United (4-1-4-1): O'Donnell; Fisher, Wood, Belaid, Mattock; Smallwood; Forde (Kelly 87), Vaulks, Adeyemi, Taylor (Allan 75); Yates (Ward 61). Subs not used: Bilboe, Warren, Wilson.
Goals: Murphy 45+2, Ritchie 49, 77, Perez 59 (Newcastle).
Referee: Jeremy Simpson (Lancashire).
Attendance: 52,208 (3,229).