Goals near the end of each half from Emily Murphy and Beth Lumsden were the difference for Tanya Oxtoby's side, who have moved level on points with Bristol City Women in third place.
The Lasses managed the game well and restricted Pompey to just two shots on target, which helped the back four record their second clean sheet of the season - the other coming in Oxtoby's first game in charge against Southampton Women.
Oxtoby made just the one change from last week's away win, handing Emilia Larsson her full debut in front of almost 2,000 United fans, while January deadline day signing Simone Charley came off the bench in the second half for her first minutes in black and white.
On a soaking wet pitch at Gateshead International Stadium, the opening 30 minutes were a close affair with neither side able to make a breakthrough or challenge the opposition's goalkeeper.
However, it was the visitors who could've taken the lead when Aoife Mannion was dispossessed by Angelina Nixon in her own defensive third. She picked out Lucy Shepherd but the midfielder let the ball hit her standing leg as the chance evaporated.
Pompey threatened again five minutes later when captain Jasmine Bull's looping cross missed everyone, including Anna Tamminen, but she watched on as it bounced off the top of the crossbar.
Despite failing to test Jess Gray in the opening 40 minutes, a moment of individual brilliance from Lois Joel helped crack open a tight Portsmouth defence.
The Wales international won the ball in the Pompey half before playing a one-two with Jordan Nobbs and delivering an inch-perfect cross for Murphy to volley home from inside the six-yard box.
United began the second half in the same fashion as they finished the opening 45 minutes, by pushing the visitors back and applying the pressure.
Murphy, looking for her second of the afternoon, challenged Gray outside her area and won the ball, but instead of shooting into an empty goal, she decided to tee up Freya Gregory, whose fizzing goalbound effort was blocked by a Portsmouth defender.
After falling behind late in the first half, Portsmouth were more often than not having to cope with waves of United pressure, but with the scoreline just at 1-0, the visitors were right in the game.
So when Lumsden, who came off the bench in the second half, was slipped through down the left and finished hard and low past Gray, relief and celebration was felt around the stadium.
Newcastle United Women: Anna Tamminen, Charlotte Wardlaw, Jordan Nobbs, Jemma Purfield, Molly Pike (Jasmine McQuade 86), Freya Gregory (Simone Charley 73), Emilia Larsson (Beth Lumsden 57), Kaitlyn Torpey, Lois Joel, Aoife Mannion (c), Emily Murphy
Subs not used: Claudia Moan, Demi Stokes, Shania Hayles
