IN PICTURES: Cambridge United 0 Luton Town 1
The Hatters forward pounced after 81 minutes, netting the rebound when his penalty was saved by home keeper Chris Dunn.
Cambridge had been reduced to 10 men just before the hour mark, when on-loan home debutant Tom Naylor was sent off after a two-footed tackle on Jim Stevenson.
Luton took control from there though and Pelly Ruddock Mpanzu, who had earlier hammered a shot against the bar, was brought down in the box, before Cullen eventually did the rest.
For a gallery of pictures from News/Gazette photographer Liam Smith, click on the link above.