Cambridge booked an FA Cup Third-Round tie with Luton after a deflected Sullay Kaikai shot after only 10 minutes saw them pull off a 1-0 second-round replay win against Reggie Lambe and his Mansfield Field Town teammates at the One Call Stadium.
But the home side were furious when Luke Waterfall's 89th-minute overhead effort was ruled out for offside when they thought they had finally pierced Cambridge's resilient defense.
Mansfield might have gone ahead with just 35 seconds on the watch but Vadaine Oliver sent a five-yard header straight at Chris Dunn from Rakish Bingham's left-wing cross.
Instead, Mansfield were rocked by Cambridge's 10th-minute goal as Kaikai's low shot from the edge of the box took a wicked deflection off Waterfall to leave keeper Sasha Studer helpless.
Oliver headed another good chance wide after 12 minutes after Ollie Palmer had nodded the ball to him at the far post.
An end-to-end half also saw Matty Blair force Studer to save with his legs after Simon Heslop was caught in possession after 38 minutes while in stoppage time Chris Clements had a powerful shot from 20 yards deflect narrowly wide.
Substitute Ryan Bird saw Studer beat away a vicious 20-yard blast on the hour on a swift Cambridge break.
Seven minutes later Lambe poked a finish inches wide for the home side.
Near the end Mansfield poured forward and Bingham had a shot deflect just wider while Matt Rhead headed over.
Just one minute from time, Waterfall thought he had forced extra-time with an overhead kick after a scramble but the linesman's flag cut short his celebrations.
