Anaisa Harney found the target for a third successive National League game to send Pompey to the top of the table.
Her effort in the opening stages at Gillingham was ultimately enough to secure all three points for the Blues.
Jay Sadler made two changes from the side that fought from behind to beat Bridgwater the previous week.
Evie Gane made a first appearance since returning to the club, while there was also a start for Leanne Bell.
The visitors made a blistering start to the contest and Bell nodded just wide following great work from Sophie Quirk on the left.
And the deadlock was broken on six minutes, with HARNEY on hand to slot home after Nicole Barrett’s long-range effort had been parried away by Cara Davies.
They then somehow failed to double their lead, as Quirk’s effort came back off the post and Bell’s follow-up was blocked.
It was not all one-way traffic, though, and Hannah Haughton did well to deny Kara Fordjour from just a few yards out.
Skipper Danielle Rowe then kept her composure to steal the ball from Amy Haynes and halt a dangerous Gillingham break.
But Pompey were still looking to get forward and Annie Rolf saw her header cleared behind after meeting Barrett’s teasing delivery.
Half Time: Gillingham 0 Pompey 1
The visitors were left breathing a sigh of relief shortly after the restart, with the Gills going close to levelling.
Breon Grant’s strike rattled the crossbar and Haughton then did brilliantly to stop Georgia Griffin converting the loose ball.
Sadler turned to his bench to freshen things up, with Ali Hall, Sammy Quayle and Dani Lane all coming on.
And the former soon linked up well with Quirk, whose close-range shot was well held by keeper Davies.
Rolf then almost wrapped things up on 77 minutes, meeting Rowe’s free-kick with a header that came back off the woodwork.
Ella Humphrey also went close deep into stoppage-time, with the substitute’s audacious long-range strike flying just wide.
But one goal proved enough for the Blues and with Oxford losing, it saw them rise to the National League summit.
Pompey: Haughton; Barrett (Quayle 65), Gane, Rowe (c), Collins; Rowbotham, Macdonald (Lane 65), Rolf; Bell (Scott 90+2), Harney (Hall 65), Quirk (Humphrey 87)