Away triumph for Millers
The match was settled four minutes into the second half when Pringle struck the only goal of the game, turning in a centre from Paul Taylor.
It was a reward for a tireless display from the Millers. They denied the home side very few clear sights of goal and the winning margin might even have been greater.
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Hide AdAlex Revell prodded a super chance wide just before the break and Pringle had a vicious 20-yarder beaten away by goalkeeper David Forde moments after his goal.
It left the home side to rue two early misses by Richardo Fuller and Mark Beevers and despite pressing forward for an equaliser, they could not find a way through a disciplined defence.
The Rotherham line-up included Anthony Wordsworth, the midfielder who joined on a long-term loan deal from Ipswich yesterday.
Millwall: Forde, Dunne (capt), Wright (Gueye 50), Williams, Beevers, Martin (Easter 72), McDonald, Malone, Edwards, Fuller (Gregory 84), Woodford
Millers: Collin, Broadfoot, Skarz (Wood 80), Morgan (capt), Arnason, Taylor, Frecklington, Smallwood, Wordsworth, Pringle (Brindley 90), Revell (Bowery 87)
Rereree: Paul Tierney (Lancs)