This result will do little to improve the health of Exiles' boss Christian Fuchs who missed his side's 21st league defeat of the season through illness.
Plucky Newport had, though, matched the second-placed Cambridge with stiff resistance, trading blows in an often physical battle for much of this rather scrappy encounter.
But moments after attack leader Courtney Baker-Richardson had so nearly stolen a lead for County, ruthless United took full advantage, two devastating defensive lapses inside three second-half minutes - ensured Liam Bennett and Sierra Leon raider Sullay Kaikai gleefully profited between the 74th and 77th minutes.
Newport's assistant manager Mark Smith must have been horrified as all Port's hard graft disappeared in a puff of smoke as a 4,000-plus gate at Rodney Parade gazed on in disbelief - the hosts crumbling as Neil Harris' charges turned up the heat.
A mixed bag of rival strugglers' results around them show County to be still only three points from League Two safety. After the trip to Fleetwood next Saturday, home matches against Tranmere and Colchester follow within seven days provide an opportunity to repair this latest damage.
County, buoyant at the start, made just the one change - enforced - to the side that won 3-1 at Salford four days earlier, former Swansea City defender Joe Thomas slotting in at right-back for the injured Liam Shephard, The Port continued with a midfield diamond, Ben Lloyd supporting a warhead of Baker-Richardson and ex-Penybont attacker James Crole.
The promotion-seeking U's, backed by around 450 fans, who had made the long journey to Gwent from Cambridgeshire, made a couple of alterations to the team held at home by Colchester in mid-week, goal-scorer Bennett and skipper Dominic Ball returning, Kaikai was to make his mark later, of course, from the bench.
Both rivals were boosted pre kick-off - County noting fellow strugglers Barrow had surrendered a two-goal lead to crash late at Fleetwood, Cambridge, initially in third spot, seeing Swindon, just above them, lose at their County Ground, in a shock 2-1 reverse to Crewe.
The contest was disjointed and frantic within the mid-pitch battle ground as both sides attempted to break the stranglehold each appeared to have. It was occasionally feisty, and United's Pelly Mpanza was cautioned for a high challenge on County's Sven Sprangler.
The visitors were creeping forward, though - Ben Knight extended Port custodian Jordan Wright at his near post in the 19th minute. On the half-hour Harris' United should have been in front - Bennett powering over from close range after Mpanza and former County linkman Shane McLoughlin had combined on the left.
A sharp counter had almost stung Cambridge moments earlier, however. Harrison Biggins and Sprangler made muscle tell, Lloyd moving between Mamadou Jobe and Kelland Watts - the youngster ultimately crowded out in the box.
United had registered almost 69% possession coming into the break, but County captain Matt Baker and co-centre back Ryan Delaney were standing firm, snuffing out any threat through Louis Appere, Knight and James Brophy - vigilant keeper Wright displaying impeccable handling at this stage from a flurry of set plays.
The Port looked to re-shuffle at the break, Cole Jarvis, a recent capture from Merthyr Town replacing Thomas, Cameron Evans moving into right-back, But there was a flash of menace from Cambridge on 56 minutes, County struggling to clear Brophy's in-swinging corner, Watts' pulling his effort wide.
Yet Newport might have smashed the deadlock moments later. Biggins' free-kick was excellence personified, Baker-Richardson moving past marker Jobe, but somehow skewing his right-footer past a post - it proved to be a pivotal moment in the clash.
Desperately seeking a maximum, United made a triple change on 61 minutes - Kaikai, Shayne Lavery and Korey Smith thrown into the fray, Mpanza, Appere and Shane McLoughlin withdrawn.
County responded by introducing Michael Spellman, Nathan Opoku and Ged Garner for Baker-Richardson, Crole and Lloyd. But the contest was swinging away from the Port within three debilitating minutes.
Newport failed to deal with James Gibbons' delivery from the left, Bennett's point-blank header squirming into the net past a distraught Wright on 73 minutes
Former Grimsby stopper Wright's misery deepened in the 76 minutes, Bennett's right-side looping cross sparked mayhem, and County's gloveman fatally mishandled in the chaos allowing Kaikai to bag his tenth goal of the campaign.
Newport County [4-1-3-2]
Jordan Wright
Joe Thomas
Matt Baker [C]
Ryan Delaney
Tom Davies
Sven Sprangler
Cameron Evans
Harrison Biggins
Ben Lloyd
Courtney Baker-Richardson
James Crole
Subs
Ged Garner for Lloyd 71 mins
Cole Jarvis for Thomas 46 mins
Shaun MacDonald [GK]
Nathan Opoku for Crole 71 mins
Michael Spellman for Baker-Richardson 57 mins
Jaden Warner
Anthony Glennon for Davies 18 mins
Cambridge United [3-4-2-1]
Jake Eastwood
James Gibbons
Mamadou Jobe
Kellard Watts
Liam Bennett
Pelly Mpanza
Shane McLoughlin
James Brophy
Ben Knight
Dominic Ball [C]
Louis Appere
Subs
Ben Hughes [GK]
Ben Purrington for Knight 80 mins
Korey Smith for McLoughlin 61 mins
Sullay Kaikai for Mpanza 61 mins
Shayne Lavery for Appere 61 mins
Elliott Nevitt for Brophy 88 mins
Sean Raggett
Referee: Lewis Sandoe
Assistants: Sean Rothwell & Ollie Williams
Fourth Official: James Durkin
Attendance 4,155 [426 ]
