Boss John Mousinho says Portsmouth's recent improvement can be traced back to the impressive performance in the defeat against Arsenal in the FA Cup at the start of January.
Pompey are now six points clear of the Championship relegation zone following the win over promotion-chasing Millwall - their third win in four matches.
"I know we lost to Arsenal [in the FA Cup], but we were excellent on the day and we've replicated those performances away from home," Mousinho said.
"I think it's come from a reset after [losing to] Bristol City, and a recognition of the fact that we can do both sides of the game pretty well when we put our minds to it.
"To come here and win, against the third best team in the league - we've watched them time and time again be a very impressive side, home and away from home - is very pleasing.
"They've got threats all over the pitch, so to do that off the back of Tuesday [win over Charlton Athletic] was really pleasing. I thought we were excellent throughout."