The striker notched his third goal of the season when following up his penalty that saw saved by home goalkeeper Chris Dunn.
The Hatters took advantage of their numerical advantage in the second period after home midfielder Tom Naylor, making his debut, was sent-off for a late challenge on Jim Stevenson.
Cambridge had the better of the first half, which saw them also strike the woodwork, but after the break it was a different story and the Town held on to record victory and third successive clean sheet.
It all started with three changes to the team that drew at York on Tuesday with skipper Stephen McNulty returning to a back three after his girlfriend gave birth to a baby girl on Wednesday. He joined Stevenson and Alex Wall in the starting XI. Jake Howells, Andy Drury and Jonathan Smith all missed out.
Cambridge, managed by former Town boss Richard Money – who also made three changes – had the better of a very turgid opening 45 minutes.
Striker Tom Elliott had the first chance for the hosts when he headed over the crossbar in the fifth minute.
It wasn’t for another 13 minutes until the next chance, again for the U’s, when Kwesi Appiah showed great strength to lay the ball off to Ryan Donaldson but his first-time shot at the post rolled wide of the target.
As the Town struggled to create an opening in attack, United came close once more. Donaldson sped upfield after Wall’s airshot at the other end and his pass found Elliott whose 10-yard drive bounced off the outside of the post.
Curtley Williams limped off and was replaced by Drury on 27 minutes in a change that saw the Town revert to a 4-4-2 formation with Alex Lacey dropping to right-back. Moments later Mark Tyler was brave to snuff out Elliott’s attempted shot in the six yard box and then Luke Wilkinson nodded away the danger when Harrison Dunk crossed dangerously from the left.
As half-time approached, both sides exchanged poor efforts from free-kicks. Donaldson whacked over his for the hosts on 39 minutes, before Wall sent a tame effort low and well wide of the target for the Town at the other end in first half stoppage time.
Cambridge came close to breaking the deadlock just seconds after the restart when Tyler kept out Appiah’s snapshot from 12 yards.
To their credit, though, the Town had started the second half brighter and their cause was helped nine minutes into the second half when home debutant Naylor was shown a straight red card for a nasty challenge on Stevenson in the Hatters half.
The game, however, had little time to flow as both sides traded yellow cards, Wall was booked for a foul on Robbie Simpson before he was substituted on the hour and his replacement, Ross Lafayette, was cautioned with a minute of his introduction for a foul on home keeper Chris Dunn.
As the Town upped the ante they came mighty close to taking the lead on 66 minutes when Pelly Ruddock Mpanzu hit the crossbar with a crashing drive from the edge of the box.
In a complete contrast to the first half it was now all the Hatters with the ball and Luke Wilkinson spurned a good chance on 70 minutes when he headed over from six yards following Luke Guttridge’s cross.
A series of corners and free-kicks into the Cambridge area were cleared and United almost snatched a goal from nothing when Donaldson’s shot was charged down well by Wilkinson.
However, with 10 minutes left, the deadlock was finally broken. The Hatters cleared their lines well from a corner, Drury feeding Ruddock who raced upfield and burst into the penalty area. He took on Richard Tait but was hauled down and the assistant referee awarded the penalty. Cullen stepped up but saw his effort from 12 yards saved, but the striker was alive to the rebound and side-footed home into the net.
Guttridge was withdrawn for Matthew Robinson immediately after the goal and moments later Cambridge came close with Elliott and Greg Taylor just off target in their quest for a quickfire equaliser.
As time ticked down, the Town played keep-ball when they had possession whilst Cambridge’s high balls into the box in search of a late, late leveller where dealt with as the Hatters held on to secure their second win in three matches.
TOWN: Tyler; Williams (sub Drury 28), Griffiths, McNulty, Wilkinson, Lacey; Ruddock Mpanzu, Stevenson, Guttridge (sub Robinson 82); Wall (sub Lafayette 60), Cullen. Subs not used: Whalley, Doyle, Walker, Justham.
ATTENDANCE: 6,533, including a magnificent 2,050 backing the Hatters.