Scott Malone levelled for the Lions on 21 minutes, before George Saville put the home side in front just before the half-hour mark.
Pompey’s hopes of a second-half comeback were thwarted after Callum Johnson was shown a straight red card three minutes after the break.
9. Gassan Ahadme - 5
(Replaced by Kieron Freeman on 52 minutes): Full of effort but struggled physically and too easily dispossessed. Photo: JPIMedia
10. Ronan Curtis - 7
Found it difficult to make inroads into Millwall defence, but will always give everything. Photo: JPIMedia
11. John Marquis - 5
(Replaced by George Hirst on 45 minutes): Couldn’t retain possession enough in a frustrating first-half display. Photo: JPIMedia
12. George Hirst - 6
(Replaced John Marquis on 45 minutes): Provided a different option to Marquis and some encouraging touches. Photo: JPIMedia