Paddy Lane hit his 10th goal of the season as Pompey won at Carlisle to strengthen their League One promotion hopes.
They had been frustrated by their opponents before the break, but were much the better side after it.
And Abu Kamara came off the bench to provide the assist for Lane to secure a positive outcome, with points dropped elsewhere in the race for the Championship.
John Mousinho made one enforced change from the side that eased to victory over Northampton on home soil the previous weekend.
Tom McIntyre was injured in that contest, so Sean Raggett came in to replace him at the heart of defence.
Kusini Yengi was named on the bench after returning from the Asian Cup, joined by Owen Moxon, who left Cumbria for the south coast on deadline day.
The hosts did not look like a side sitting bottom of the table and showed plenty of intent to get forward.
Where they were perhaps lacking a bit of confidence, however, was in the final third, as several moves broke down before Will Norris could be tested.
And there were also attempts from Jack Diamond and Jordan Gibson that were fired wastefully off target.
Pompey took time to grow into the contest, although an early angled attempt from Lane did at least test Harry Lewis’ handling.
There was 34 minutes on the clock before either side created an opportunity that seriously threatened to break the deadlock.
It fell to the visitors, as a Marlon Pack corner was cleared as far as Tom Lowery, whose effort from the edge of the box was diverted against the post by Lane.
Moments later, there were no passing options immediately obvious for Raggett, so instead he hit a 30-yard shot that dipped narrowly over the bar.
But neither side had truthfully done enough to be disappointed about heading in all square at the break.
Half Time: Carlisle 0 Pompey 0
The Blues looked to get at their opponents right from the restart and Joe Rafferty soon skipped past Jon Mellish.
He sent a low delivery towards the back post, where Colby Bishop was waiting, but Sam Lavelle did well to turn it behind.
Connor Ogilvie then found an unmarked Myles Peart-Haris inside the box, only for the usually composed loanee to let the ball run underneath his foot.
There was much more purpose from the visitors and Lane soon screwed a shot wide as the pressure grew on United.
Mousinho freshened up his forward line with a change on 57 minutes, as Peart-Harris made way for Kamara.
And it soon had the desired effect, with the forward neatly controlling the ball on the right, beating his man and driving inside.
He then threaded a perfect pass through for LANE, who took it in his stride and confidently slotted home.
Pompey were in no mood to sit on their lead and when Lewis dropped Lane’s cross under pressure from Conor Shaughnessy, the grateful keeper was able to grab the ball at the second attempt.
The keeper then got in the way of Bishop’s cut-back, which bounced up and was hooked over by Callum Lang.
That was the January signing’s last taste of action, however, as he made way for Moxon, who received a warm welcome from both sets of supporters.
It was still the visitors looking more likely to add to the scoring and when Pack swung in a free-kick, Shaughnessy headed against the post.
That was soon after Carlisle had made a quadruple substitution in an attempt to find a way back into the contest.
But the Blues saw out the closing stages in comfortable fashion to make the long journey home that much sweeter.
Carlisle (4-1-4-1): Lewis; Back (Grehan 75), Lavelle (c), Barclay, Robinson (Armer 75); Neal (Maguire 75); Gibson (Charters 75), Vela, Mellish, Diamond (Butterworth 57); Armstrong
Subs not used: Breeze, McCalmont
Pompey (4-2-3-1): Norris; Rafferty, Shaughnessy, Raggett, Ogilvie; Pack (c), Lowery; Lane (Sparkes 85), Lang (Moxon 75), Peart-Harris (Kamara 57); Bishop
Goals: Lane 62
Subs not used: Macey, Towler, Martin, Yengi
Referee: Martin Woods
Attendance: 9,053 (1,848 Pompey fans)