United had the better of the opening 45 minutes, asking a few questions of their opponents' defence without truly troubling the Bluebirds.
Leeds certainly looked to grow in confidence as things wore on, though, coming close to making a breakthrough when Stuart Dallas burst through and fired wide from 25 yards out.
Match-winner Mowatt - back in the starting lineup this evening as one of three changes - also fired in a shot, while Dallas saw his latest attempt kept out by David Marshall.
The final opportunity of the goalless first half fell the way of Chris Wood, who headed Bellusci's deep cross wide of goal following what had been a largely uneventful opening 45 minutes.
Perhaps unsurprisingly judging by both sides' goal stats of late, the second half largely followed a similar pattern as a scrambled Cardiff corner was all the game had to show in terms of excitement in the 15 minutes following the restart.
Yet Leeds did finally manage to break Cardiff's resilient defence, which had not conceded in each of the last four games heading into this one, with Mowatt smashing a 25-yard strike beyond the reach of Marshall.
Marco Silvestri had to keep out a couple of efforts, although it was the hosts who perhaps should have bagged a late second when Wood rounded Marshall but could only pick out the side-netting.
Leeds did manage to see things through in the end, however, to record a first triumph over their opponents in more than three decades.