Promotion-chasing Cambridge defied the odds to extend their unbeaten run to seven matches.
United were down to nine men and trailing 2-1 when manager John Taylor introduced substitute Paul Wanless with just 14 minutes left. The midfielder, playing against his old club, produced a dramatic equaliser just two minutes after his entrance.
Lincoln defender Paul Morgan failed to clear a free-kick and although Wanless' first shot was blocked he was quick to hit the rebound into the net.
Lincoln produced some bright football early on with Paul Smith wasting a clear-cut second minute chance when he shot wide from ten yards.
Cambridge were quick to punish the home team, going in front with a breakaway goal just two minutes later. Striker Dave Kitson fired in a shot which was pushed out by Lincoln keeper Alan Marriott, but Luke Guttridge followed up to put the ball into the empty net.
Lincoln went close to an equaliser in the sixth minute when loan signing Steve Watts beat the keeper with a low shot only to see Cambridge midfield man Francesco Nacca clear off the line.
The equaliser eventually came in the 34th minute when a long-range effort from Ben Sedgemore was flicked in by Richard Butcher.
Butcher then produced an excellent cross only for Cambridge keeper Shaun Marshall to produce a diving save to keep out Mark Bailey's header.
Cambridge were reduced to ten men in the 65th minute when Terry Fleming received his second yellow card for a challenge on City sub Chris Cornelly.
Cornelly, a midweek signing from non-league Ashton, shook things up and set up a 67th minute goal for Simon Weaver.
Cambridge then had Guttridge red-carded for a challenge on Smith, which left the Lincoln man laid out. Referee Eddie Evans produced his red card for a third time in the 88th minute when Lincoln's Scott Willis was dismissed for a head-high challenge on Wanless just four minutes after coming on.