The midfielder tapped home after 49 minutes to ensure that the Millers made it two wins from four league games since their promotion in May.
Alex Revell could have given them a first-half lead when he shot wide from six yards, but Pringle's strike along with some excellent defending proved to be enough.
The result, meanwhile, was Millwall's first loss of the season, and the first time that they had tasted defeat in the Championship since March.
The visitors had the first effort of the game on four minutes when new signing Anthony Wordsworth sent a looping volley just over the crossbar.
Millwall had claims for a penalty waved away two minutes later when Mark Beevers appeared to be wrestled to the ground by Kirk Broadfoot.
The ball ricocheted out for a corner and from the resulting set-piece the defender was fortunate to see the ball deflect off of him and narrowly wide.
On 14 minutes Millwall winger Lee Martin forced an important block from Craig Morgan as he met a low cross from Carlos Edwards.
Undeterred, the Lions wideman then found possession on the left and delivered a delicious cross that Ricardo Fuller could only glance wide.
Millwall were dominating possession but, in a rare Rotherham break forward, Paul Taylor rode two challenges on the right before firing off target after 30 minutes.
And it was the Millers who forced the best chance of the half when Revell missed an open goal.
Broadfoot had seen a close-range volley blocked before Taylor unleashed a strike for the edge of the box. Forde managed to palm it to his right but only to Revell, who poked wide of an unguarded net.
Taylor, who had grown into the game, sent a fierce half volley into the side netting 90 seconds after the break, and they opened the scoring two minutes later.
Taylor floated the ball into the area and Pringle was well placed to tap in from four yards.
Adam Collin spilled Shaun Williams' near-post drive on 55 minutes and the Rotherham defence managed to scramble clear.
Pringle almost grabbed his second of the game a minute later but Forde did superbly to beat away his 20-yard drive.
Fuller could have levelled for the Lions after 64 minutes but he could only head over from Scott Malone's deep cross.
Broadfoot then headed an excellent Pringle free-kick over, before Martin shot well wide for Millwall, as the game entered its final quarter.
Millwall substitute Magaye Gueye had a volley well blocked on 80 minutes but Rotherham held firm to take all three points.