The Imps started brightly, putting the home defence on the back foot early on although United 'keeper Chris Maxwell was untroubled, although a number of throw-ins were causing a headache or two for his team mates.
City carved out the first half-chance of the game in the 10th minute with Alan Power having a free-kick claimed by Maxwell whilst three minutes later Tomlinson tested the home stopper with a low drive from 25 yards.
A quiet spell ensued with play centered mainly in the middle third as both sides seemed to cancel each other out towards the half-hour mark; a slight scare for the Imps coming on this 30 minute mark when a neat chip from Luke Berry released Delano Sam-Yorke but the offside flag saved City, although Paul Farman got in a great save to deny the big striker.
City were certainly having the better of the possession as the half wore on but an interruption to allow treatment for Adam Cunnington following an aerial challenge with Nat Brown halted the momentum a bit until a sweeping attack in the 38th minute ended with Newton winning a corner which ended up seeing Jon Nolan have a shot hacked away.
Play immediately switched to the other end with Cambridge wasting their first corner of the game but it wasn't long before the hosts were pressing again and, after Nolan was unable to clear his lines following a cross into the box, the ball ran kindly for Berry and he duly blasted past Farman and into the roof of the net from 12 yards to give his side the lead.
City looked for a quick response with a Ben Tomlinson cross being cleared off Sheridan's feet by a home defender and Sheridan was then inches away from connecting with a Newton cross as the Fourth Official indicated that a minimum of two minutes were to be added on.
HT: Cambridge 1 Imps 0
Whilst City emerged unchanged, Cambridge made a switch prior to the start of the second period with Greg Taylor coming on for Delano Sam-Yorke.
The Imps started encouragingly with Newton having an effort blocked before he drilled over after Jordan helped on a throw-in whilst at the other end Ryan Donaldson dragged a low effort a foot wide from the edge of the box.
Newton and Jordan combined again in the 57th minute to win a corner on the left but Alan Power's inswinger was punched away by Maxwell. Another corner followed on the hour mark with this ending up with Foster having an effort blocked.
Sheridan and Cunnington then squared up close to the touchline with the outcome being a free-kick to the Imps which ended with Foster lifting an effort over the bar.
City kept pressing and pushing forward with Sheridan and Miller combining to win another corner in the 70th minute and before this could be taken, Bohan Dixon and Danny Rowe came on for Jordan and Sheridan.
When the corner was taken, Newton's delivery was tipped over by Maxwell and when the follow-up flag-kick was half-cleared, Rowe's first touch saw him fire well over. City remained on the offensive and after some neat build-up play across the edge of the box, Maxwell held a rasping Nolan drive.
Mitch Austin for Harrison Dunk was United's second change of the game and shortly after this Rowe was denied by a superb Maxwell save low to his right. In a bid to calm things down the home stopper went down holding his head, much to the annoyance of the travelling City fans behind his goal.
After Arnold went down easily in the box following a rare Cambridge attack, Tom Miller was cautioned for a late challenge on Austin. Michael Spillane replaced Cunnington before the free-kick was taken.
Spillane's first taste of the action saw him drill wide from long range before the Fourth Official indicated that a minimum of four minutes were to be added on.
Berry curled over as he looked for a second but drama ensued midway through the extra stages when Power slid on on Taylor for a 50-50 ball. The United players were quick to surround and put pressure on the officials and once referee Kinseley consulted with his assistant was quick to brandish the red card to the City skipper.
The clock continued to tick down but despite enjoying the better of possession and chances, City slipped to a narrow defeat with Berry's first half strike proving enough to preserve Cambridge's unbeaten record.
FT: Cambridge 1 Imps 0