A dreadful error by experienced centre-back Stevland Angus sent struggling Cambridge on their way to their fourth home defeat and into the relegation zone.
Young goalkeeper John Ruddy pulled off five excellent saves from five different Lincoln players to keep his side on course for what looked like being a much-needed draw.
But in the 78th minute Angus failed either to clear the ball or shield it when it was knocked into the box on the left allowing Francis Green to nip in to chip it over the helpless keeper.
Manager Herve Renard brought in two strikers for their home debuts but it made little difference to a team that had managed only five goals in seven managers in front of their frustrated fans. There were only two real goal attempts from the home side.
Andy Duncan forcing an excellent save from goalkeeper Alan Marriott with a powerful ninth-minute header, while Richard Hodgson hit in the bar with a 55th-minute shot.
There was much more action down the other end where Ruddy did especially well in the 35th minute to block a blaster from Martin Curruthers and then deflect the follow up effort from Simon Yeo.
Ruddy made another superb stop to keep out Francis just after the restart, but once the gift goal was presented Cambridge fell away and was lucky not to lose by more as well organised Lincoln clinched their fourth away victory.