The 19 points that Nuneaton conceded in the ten minutes when winger Aaron Takarangi was in the sin-bin effectively cost them the match, leaving new coach Mike Umaga's team still seeking their first victory.
Otherwise it was a close contest with Nuns, especially in the second half when the wind was behind them, seemingly in total control at times although lacking Cambridge's clinical finishing.
Nuns old boy Gareth Cull put the home side ahead with an eighth minute penalty but a concerted handling attack by Nuns forwards was rounded off from 20 yards by lock Ben Griffiths with a try converted by Huw Thomas.
Cull's second penalty reduced Nun's lead to 7-6 before the defining moment came after 34 minutes as former Pertemps Bees and Coventry winger Takarangi was dismissed for deliberate obstruction and Cambridge were awarded a penalty try converted by Cull.
Two minutes later Cambridge exploited their one man advantage to set up an overlap for centre James Shanaham to score an unconverted try and on the stroke of the interval scum-half Sean Liebenberg nipped over from the base of a ruck five yards out, Cull converting.
Despite their territorial domination in the second period Nuns only reward were two penalty goals from Thomas and with ten minutes to go they shot themselves in the foot by tapping a penalty and losing possession.
Cambridge mounted an immediate fast counter-attack and replacement flanker Tom Kirkman scored their fourth try, converted by Cull, leaving a score line which did not reflect the match.