The Hatters suffered their first home defeat of the campaign and their second loss in succession as Gary Brabin's former club snatched a narrow victory.
The Town dominated the first half with visiting goalkeeper Danny Naisbitt doing well to keep out Danny Crow's goal-bound header.
United had chances too, Mark Tyler called into a stunning stop to thwart Tom Shaw's effort as the game swung end-to-end.
However the telling moment of the match came on the hour when the Town were caught napping at the back which allowed Rory McAuley to loop a header past Tyler at the back post.
Try as they might to find a level it didn't come and Brabin's side were left to contemplate a second defeat in three days.
Town boss and former United manager Brabin made two changes to the side that lost 3-0 to York with top scorer Amari Morgan-Smith and ex-Cambridge man Crow returning to the starting XI with Alex Lawless suspended and injured dropping out all together and JP Kissock on the bench.
Visiting chief Jez George included former Hatter Rossi Jarvis in midfield and striker Ryan Charles on the substitutes' bench.
After an open but quiet beginning it was the Town that asserted the early pressure. A fine one-touch move saw Cambridge cleare the danger after Morgan-Smith's right-wing cross.
On 11 minutes the Hatters should have had the ball in the net when Robbie Willmott flew down the left wing, nutmegged marker Kevin Roberts and finding Stuart Fleetwood who wriggled into the area and passed across the face of goal, but both neither Crow and Morgan-Smith were on hand to knock home.
Two minutes later, as the Town found their feet, Crow and Morgan-Smith both went close to breaking the deadlock. Willmott's superb cross from the left was glanced goalwards by Crow but his header was clawed away brilliantly by Naisbitt. The ball, however, broke at the feet of Morgan-Smith from the rebound but, from a tight angle, the forward slammed a shot into the side-netting.
Cambridge were slowly creeping back into things without testing Tyler and the Hatters again had a sight of goal on 24 minutes when a cross from James Dance was dummied intelligently by Crow but Fleetwood could only steer a first-time shot high over Naisbitt's goal.
There was almost another sniff of goal for the Town on 28 minutes as Janos Kovacs's super cross-field pass found Morgan-Smith on the right but he saw his shot from the corner of the penalty area charged behind for a corner.
Tyler had been a virtual spectator up until this point but he was called into action twice in three minutes as Cambridge grew in confidence.
Firstly on 32 minutes he made routine save to stop Jarvis's piledriver before producing a sensational flying save to keep out Shaw's blockbuster effort on 25 yards that was zooming fast into the top corner.
The Hatters were playing some decent stuff in the final third as half-time approached, Morgan-Smith and Crow combining well again that almost put the former in on goal but he was denied by a last-ditch interception.
Cambridge started the brighter of the two sides after the break, Luke Berry twice calling Tyler into action in the space of 60 seconds; firstly flinging out an arm to divert a shot wide at his near post before pouncing on a shot by the visitors' number 18 from 18 yards.
Morgan-Smith twisted and turned down the left on 51 minutes that saw Howells shoot high and wide after the U's had half-cleared before Peter Winn and Shaw both fluffed chances for the visitors a minute later.
There was no doubt that Cambridge had started the second half the better and they took the lead on the hour, the two centre-backs combining: Josh Coulson's right-wing cross was met by a looping header by McAuley at the back stick, his effort arching in slow motion beyond a helpless Tyler to send the travelling support into raptures.
The Hatters were struggling to find their passing game as the visitors frustrated Brabin's men - and Cambridge seemed content with their one-goal advantage.
With 13 minutes left on the clock John Paul Kissock replaced Willmott as the Hatters went 4-3-3 with Kissock playing an advanced role in a midfield trio.
Adam Watkins joined the fray six minutes before the end as the Hatters looked for a way back into the game.
But in truth the Town rarely looked like troubling Naisbitt in the second half - the closest they came to forcing an equaliser came four minutes from time when Morgan-Smith blazed over following a half-cleared corner and, after four minutes of stoppage time bore no fruit, the Town left reflecting on another defeat.
It will have hurt big time - for Brabin in particular - and the Town boss will need to raise his troops on Saturday for the visit of Barrow.
Get down here, back the boys and keep the faith.
For minute-by-minute analysis see below.
Result from Kenilworth Road
Luton Town 0
Cambridge United 1 (McAuley 60)
FULL-TIME All over here, reaction and photographs to follow as the Town fall to their first home defeat of the season.
90 mins - Four added minutes for the Town to find a late equaliser.
86 mins - Town pressing but low corner, half-cleared by the visitors is met by Morgan-Smith but his effort is always rising as it find the Kenilworth Road end.
85 mins - Into the last five...
84 mins Last throw of the dice for Brabs as he sends on Adam Watkins for Jamie Hand.
80 mins - Into the last 10 minutes for Town to get something from this game.
79 mins - Kissock's arrival sees the Hatters go 4-3-3 with Kissock in central mifield with Keane and Hand, AMS and O'Connor in forward positions and Fleetwood down the middle.
77 mins Second change for the Town, Kissock replacing Willmott. United make their second too, Harrison Dunk replacing Winn.
75 mins - Into the last 15 minutes here.
70 mins - The attendance is 6,274 which includes 522 supporting Cambridge.
67 mins - First booking of the night, for Roberts on Willmott.
65 mins - Town looking for a way back into things here but Naisbitt not called into action this half.
60 mins GOAL! Town 0-1 CAMBRIDGE (McAuley) - Oh dear, a ball from the right flank catches the Town cold and, short of cover, RORY McAULEY heads sends a looping header arching into the top right hand corner giving Tyler no chance.
58 mins Town make their first change, introducing Aaron O'Connor
for Crow. Cambridge also make their first sub too, Ryan Charles replacing Gash.
55 mins - Cambridge pressure now as they look to break the deadlock...Firstly Peter Winn drives down the left to put a cross across the face of goal before Shaw, who had the great first half effort saved, gets a first time shot all wrong after the ball had broke to him 18 yards out.
51 mins - Great work down the left by Morgan-Smith eventually sees him cross which is half-cleared by Cambridge to Howells who lets fly with a shot that goes just high and wide of Naisbitt's goal.
50 mins - Half chances for the visitors as they press for an early goal in this second half. Firstly Tyler stops a Luke Berry shot from creeping in at his near post before the keeper denies the same player from 18 yards.
46 mins Annnnnnnd we're off again as the Town restart, shooting towards the Kenny End...no changes for either side at the interval.
HALF-TIME Goalless here at half-time at Kenilworth Road, check back in 15 minutes for live updates. COYH.
45 mins - Pretty even here, one minute added at the end of the first 45.
40 mins - Five minutes to the break, Town pressure before the break.
34 mins - Another shot on goal and this time Tyler is extended fully as he flings himself to his right to keep out Tom Shaw's great hit from 20 yards that was charging towards the top corner. Absolutely top save.
32 mins - First meaningful action for Mark Tyler tonight as Rossi Jarvis tries his luck from 25 yards with a shot that the Town stopper deals with easily.
28 mins - Pressure rising again, Kovas spreading play brilliantly to Morgan-Smith on the right flank, he cuts inside and shoots but sees a shot deflected behind by Rory McAuley. However, nothing comes of the resultant corner.
24 mins - Almost a sight of goal for Fleetwood. Dance crosses deep from the right, Crow dummies into the path of Fleetwood who strikes first time over the bar.
20 mins - Twenty on the clock and still goalless, Cambridge have ridden that storm by the Town, creeping back into things slowly.
13 mins - Chance. And it's almost 1-0. Willmott centres accurately for Crow to glance a header goalwards that looks to be heading for the bottom corner until Danny Naisbitt in the visiting goal gets down to turn the ball away. With the goal gaping the ball falls to Morgan-Smith but, from a narrow angle, the Town top goalscorer can only find the side netting.
11 mins - Better from the Hatters as they begin to up the pressure. Willmott does brilliantly to nutmeg Kevin Roberts down the left and find Fleetwood. He then beats his man in the area, drives on and squares a perfect ball across the six yard box but neither Crow or Morgan-Smith are there to turn the ball home.
8 mins - Almost a sniff of a chance for the Town as good football down the right leads to Morgan-Smith crossing low to the near post but Cambridge manage to clear.
5 mins - Very even start here at Kenilworth Road, as expected James Dance at right-back, Keith Keane and Jamie Hand in central midfield, Fleetwood in attack with Crow.
1 min&nbsp; And we're off, enjoy the updates. COYH.
PRESS BOX PREDICTIONS
Andrew Barringer: 3-1 Luton, Simon Pitts: 2-0 Luton, Mike Simmonds: 1-1, Mark Wood: 2-0 Luton, Bill Cole: 2-1 Luton, James Cunliffe: 1-0 Cambridge
19:20 EX-HATTERS, EX-U'S
The Hatters have ex-U's Danny Crow and Robbie Willmott in the starting XI, and manager Gary Brabin and first team coach Paul Carden on the subs' bench. Cambridge meanwhile have former Town man Rossi Jarvis in midfield and striker Ryan Charles on the bench. The Jordan Patrick amongst their subs is not the former Town youngster.
19:15 ON THIS DAY
The Town's record on this day is Played 17, Won 8, Drawn 5, Lost 4, Scored 31, Conceded 23.
The last game was at Kenilworth Road in 2008 when the Hatters drew 1-1 with Chester City. Asa Hall scored the Town goal that day.
19:00 TWO CHANGES
Danny Crow returns to the starting XI against his old club alongside Amari Morgan-Smith. Both come in for Alex Lawless and JP Kissock.
Town: Tyler, Howells, Antwi, Kovacs, Dance, Hand, Keane, M-Smith, Willmott, Crow, Fleetwood. Subs: K. Pilkington, Asafu-Adjaye,&nbsp;O'Connor, Watkins, Kissock.
Cambridge: Naisbitt, Roberts, Jennings, Coulson, Shaw, Gash, Carew, McAuley, Jarvis, Berry, Winn. Subs: Charles, Murtagh, Dunk, Patrick, Yakubu.
18:30 TEAM NEWS IN 30
Standby for team news in the next half-hour.
17:40 WHAT WE NEED
Just bumped into Gary Brabin in the tunnel..."We need some bouncebackability tonight," he says.
17:30 WEATHER WATCH
It's a beautiful Autumn evening in Luton. Sunshine and not a cloud in the sky.
The true test of a great team is how they react to disappointment.
Those will be the words ringing around the Hatters' dressing room minutes before kick-off as Gary Brabin briefs his side one last time before they take on Cambridge United this evening.
Saturday's 3-0 defeat to York was a kick in the teeth, a disappointment and, perhaps, a reality check. Brabs was despondent following such a set-back. "I could see it coming," he said. "This is a set-back and we need to get back on the front foot against Cambridge."
The Town don't lose many by such a scoreline. The last time and they lost 3-0, and indeed the only time in the Blue Square Bet Premier they have lost by three clear goals, was against Wrexham in September 2009.
That Tuesday night in North Wales was one to forget and reaction was needed. Four days later the Hatters did precisely that.
And here comes the hint of irony. The team on the receiving end of the reaction were Cambridge United.
A 4-3 victory at the Abbey Stadium erased memories of the 3-0 defeat to Wrexham, not that they did it the easy way...2-0 down and a man down at half-time before a second-half revival took hold.
Brabs will be hoping for a similar, if less dramatic, response against his former side under the Kenilworth Road floodlights tonight.&nbsp;
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