Stoke captain Joe Allen guided his team to a vital win and finished Cardiff’s seven-game unbeaten streak as he took his team away from the relegation’s position.
Ex-Swansea player Allen converted Tyrese Campbell’s assist to score Stoke’s second goal in the 72nd minute and boosted his team’s chances to secure an important three points.
Callum Paterson opened the scoring in the 25th minute after he deflected a corner from the man of the match Jordan Thompson into his own net.
Delighted Stoke boss Michael O’Neill said: “I think we showed a lot of resilience today and that’s something that has been questioned this season on a number of occasions so it was very good to show that today against a team that were always going to test you.
“We had to deal with a lot of balls into the box and direct play which I think we dealt with well and we created some good chances at times.
“I felt that we were never really under any great threat and obviously looked a threat ourselves.”
Campbell could have extended the lead on three occasions, but Alex Smithies saved brilliantly.
The Bluebirds had few chances to score, the main one coming from striker Danny Wards in the 18th minute.
The Potters had the highest home attendance, this season, 25,436, with 1,405 Cardiff supporters adding to the atmosphere.
Millwall boss Neil Harris said: “It was hugely disappointing from the first whistle until the last. We were nowhere near the standards and levels we’ve set individually and collectively.
“We gave two poor goals away and I pride my teams on being really tough to beat and if you can’t win games then you have to make sure you don’t concede and lose them. It’s just not good enough.”