Colby Bishop found the target as Pompey won at Cambridge to record three successive League One victories for the first time in almost six months.
It did not look a likely outcome following a poor first half performance that saw them fortunate to go in level at the break.
But they did improve following the restart and Bishop nodded home an Owen Dale cross to reward the noisy travelling fans.
John Mousinho made one enforced change from the side that beat Bolton on home soil earlier in the week.
With Ronan Curtis injured, he was replaced by Tottenham loanee Dane Scarlett wide on the left.
But there was a return to the bench for Reeco Hackett, now recovered from the back problem that saw him miss the previous two games.
Michael Morrison started for Cambridge against his former club, with Steve Seddon named among the substitutes.
Pompey saw a lot of the ball in the early stages, but were unable to turn that into any meaningful chances.
Indeed, the opening half was short on quality, with both sides guilty of losing possession far too easily.
It was Cambridge who looked more likely to break the deadlock, although they failed to punish their opponents for some sloppy play.
Jack Lankester sent a low effort straight into the arms of Matt Macey before Liam Bennett headed James Brophy’s cross wastefully wide.
Dimitar Mitov misjudged Joe Rafferty’s long ball forward at the other end, but was eventually able to gather it.
Pompey were still struggling to find any fluency and might have fallen behind when Morrison had a free header from Conor McGrandles’ corner, only to send it over the bar.
McGrandles had a chance of his own after making space on the edge of the box, then dragging his shot wide.
The midfielder also sent Sam Smith racing clear and, perhaps under pressure from the advancing Macey, he fired wide.
Half Time: Cambridge 0 Pompey 0
Cambridge fired another warning shot straight after the break, although Macey was not overly troubled by Joe Ironside’s attempt.
Bennett also bent the ball wide after being found in space moments later, as the Blues continued to look well below their best.
A Rafferty delivery from the right created an opening at the other end, but Morrison did well to stop Bishop converting it.
And – backed by non-stop singing in the away end – they soon started to apply some pressure of their own.
Mousinho made a change on the hour mark and there was a welcome return to action for Tom Lowery, with Michael Jacobs the man to make way.
He soon played a lovely ball over the top for Dale and his shot was beaten away by Mitov, with the ball not falling for a Blues player.
But a goal did arrive on 65 minutes, when a corner was worked to Dale and his cross was headed home at the far post by BISHOP.
Cambridge had the ball in the net themselves soon after and it took a while for their supporters to realise a flag was raised for offside.
A fit-again Hackett replaced Scarlett for Pompey, while the home side’s substitutions included former Fratton loanee Seddon.
And he might have found an equaliser on 80 minutes, only to see his low shot well blocked by Sean Raggett.
United committed men forward in search of an equaliser as the seconds ticked down, but the Blues held firm to secure all three points.
Cambridge (3-4-1-2): Mitov; Morrison, Bennett, Jones; Bennett, Digby (Tracey 77), Lankester (O’Neil 68), Brophy (Seddon 78); McGrandles; Smith, Ironside
Booked: Lankester
Subs not used: Mannion, Williams, Okedina, Worman
Pompey (4-2-3-1): Macey; Rafferty, Raggett (c), Towler, Ogilvie; Tunnicliffe, Morrell; Dale (Pigott 89), Jacobs (Lowery 60), Scarlett (Hackett 78); Bishop
Goals: Bishop 65
Booked: Dale
Subs not used: Steward, Bernard, Thompson, Lane
Referee: Andrew Kitchen
Attendance: 6,667 (1,466 Pompey fans)