It was not the most free-flowing of performances, but it was still the hosts who created most of the game’s best chances.
And – after the past two games yielded a solitary point – there was a sense of relief when McNulty opened the scoring just before the break.
United failed to cause Ryan Fulton many problems and it was left for Webster to secure all three points midway through the second half.
That was despite a few late nerves when Cambridge skipper Luke Berry pulled one back in the closing stages.
Paul Cook made four changes from the side that were beaten at Barnet during the week.
Conor Chaplin was handed his first start in more than four months, while Christian Burgess returned after serving a one-match ban.
Kal Naismith and a fit-again Danny Hollands featured in midfield, with Matt Clarke and Ben Close, as well as the injured duo of Adam McGurk and Michael Smith, making way.
The hosts made a bright start and Chaplin fired wide from a tight angle after both Kyle Bennett and Naismith had their paths to goal blocked.
Leon Legge then did well to cut the ball out after McNulty stole possession and tried to thread a pass through to Chaplin.
But the contest quickly descended into a scrappy affair, with a number of niggly challenges drawing a shrill whistle from referee Oliver Langford.
Jimmy Spencer was finally shown a yellow card after one foul too many, while Ben Davies quickly followed him into the book.
Cambridge then had their best chance of the half, with Ben Williamson’s low cross going just behind the onrushing Berry.
But it was the Blues who broke the deadlock on 41 minutes and, unsurprisingly, it came from a free-kick after McNulty had been upended.
Michael Doyle slipped the ball through to McNULTY, who brought it under control before rocketing a shot into the roof of the net.
Spencer was then slightly fortunate to remain on the pitch following a foul on Enda Stevens, with United boss Shaun Derry preparing a substitution before eventually deciding to wait until the break.
Cambridge United 0
As expected, Spencer did not return for the start of the second half , with the striker wisely making way for Ryan Donaldson.
But it was the Blues who were first to threaten after the restart, with Bennett dragging a shot narrowly wide.
The half took a while to spark into life, however, with neither side looking particularly creative going forward.
But Pompey began to pick up as time went on and Chaplin was not quite able to pick out McNulty with a low cross.
Hollands’ drive was then kept out by Cambridge keeper Will Norris following a neat cut-back by Stevens.
And the hosts did double their advantage just past the hour mark to relieve any tension around the ground
It was Webster who started and finished the move, bringing the ball out of defence before finding Bennett.
The winger was unfortunate not to score himself with a fierce strike that Norris expertly tipped onto the crossbar.
But WEBSTER was following up to hook the ball home and send the Fratton faithful wild with delight.
Cook was forced into his first change on 73 minutes, with Naismith picking up a knock and being replaced by Gareth Evans.
Cambridge did reduce the deficit late on when a free-kick was headed back by Mark Roberts for Berry to score.
But the Blues saw out five minutes of stoppage-time to secure the win and move back into the League Two play-off places.
Pompey (4-4-2): Fulton; Davies, Burgess, Webster, Stevens; Naismith (Evans 73), Hollands, Doyle (c), Bennett (Tollitt 90+2); Chaplin (Clarke 87), McNulty
Goals: McNulty 41, Webster 62
Booked: Davies, McNulty, Chaplin
Subs not used: Bass, Freeman, Haunstrup, Barton
Cambridge (4-1-4-1): Norris; Furlong, Legge (Roberts 85), Coulson, Haynes; O’Neill (Simpson 56); Williamson, Berry (c), Ledson, Dunk; Spencer (Donaldson 46)
Goals: Berry 90
Booked: Spencer, Berry, Legge
Subs not used: Beasant, Omozusi, Keane
Referee: Oliver Langford
Attendance: 15,425 (587 away fans)