The centre-back turned a low right-wing cross from Tony McMahon into his own net from six yards to settle a poor game of few clear-cut chances.
City were booed off by their supporters at the end, having seen their methodical approach play hit a brick wall in the shape of United central defenders Harry Maguire and Neil Collins.
The only clear chance of the first half fell to City skipper Sam Baldock as he ran on to a 36th-minute pass from Jay Emmanuel-Thomas. With Scott Wagstaff better placed square of him, the striker elected to shoot and was denied by goalkeeper George Long.
Emmanuel-Thomas shot narrowly wide following a 19th-minute free-kick and Nicky Shorey fired over for the hosts from the edge of the box.
United’s only attempt was a 31st-minute free-kick from McMahon, comfortably saved by City keeper Elliott Parish.
The visitors threatened more after the break without testing Parish, while City had a late Karleigh Osborne strike ruled out for a foul.