The young Bluebirds produced their best performance of the season to earn their first Professional Under-18 Development League win.
Ntazana Ligendza, Kieron Evans and Isaak Davies scored Cardiff City’s goals during a convincing success.
Coach Matthew Bloxham was delighted with his young players, who led 2-1 at half-time and maintained a tight grip on the game.
Norwegian teenager Niklas Haugland, aged 17, replied for Leeds.
Cardiff’s next two PDL under-18 fixtures are away, against Coventry City and Ipswich Town. They are then at home against Charlton Athletics on Saturday, October 5th.
Rubin Colwill went close after five minutes, when he tested Welsh junior international goalkeeper Matthew Turner with a sublime curling effort from range.
Former Pontardawe Academy player Turner, a 17-year-old who attended Burry Port School before heading north to Yorkshire, was coached by Glan Letheren of Top Catch GK, who signed for Leeds at the same age during the 1970s.
At the other end, Australian goalkeeper Jamie Cogman made a big save to deny Leeds striker Henry Kumwenda.
Cardiff took the lead when Ligendza netted from close range after 17 minutes and the same player went close again when his powerful strike was tipped over the bar by Turner.
Haugland equalised just before the half hour, while Cogman saved superbly from a Jimiel Chikukwa shot.
Evans regained the lead for Cardiff when he fired into the bottom-left corner from 20 yards, while the same player had a shot blocked soon after.
Colwill went close with two shots, while Davies completed the 3–1 scoreline when he finished in clinical style after a superb.
Leeds United u-18: Turner, Kachosa, Stanley, Kennah, Cresswell, Burlace, Chikukwa, Blunt, Kumwenda, Gibbon, Haugland. Subs: Bray, Amissah, Rigby, Leverett.
Cardiff City u-18: Jamie Cogman, Owen Pritchard (Aidan McNamara 46), Sam Parsons, Trystan Jones, Joshua Rippon, Tom Chiabi, Kieron Evans, Harry Pinchard, Isaak Davies, Rubin Colwill (Cian Ashford 87), Ntazana Ligendza (Frazer Thomas 77). Sub not used: Brad Stewart (gk)