Full-time from Princes Park
John Marquis was on target for Millwall U21s but the young Lions were undone by a double from Cardiff City's Danny Johnson in Monday night's Professional Development League 2 match at Princes Park.
The first half saw the young Lions enjoy the majority of possession after the visitors had carved out a couple of early chances.
Lions keeper Tom King did well to deny Johnson with a smart reaction save from the Cardiff player before a free-kick from Tom James' was cleared, with Johnson heading the loose ball over the bar.
Millwall began to get into their rhythm, however, and on the quarter-hour mark Kyron Farrell's thumping drive was superbly blocked by James.
Marquis directed a header wide of the mark whilst Alfie Pavey was a whisker away from firing Millwall ahead midway through the first-half, but Cardiff keeper Ben Wilson did well to gather the striker's shot at full stretch.
The visitors enjoyed a sustained spell of pressure early in the second-half and it took a couple of smart saves from King to keep out Guido Burgstaller and then a curling effort from Kadeem Harris.
However it was Millwall who made the breakthrough just past the hour-mark courtesy of Marquis' shot that took a slight deflection on its way into the back of the net beyond the reach of Wilson.
The lead was short-lived and barely 70 seconds later Cardiff had pegged Millwall back when Jazz Barnum-Bobb laid the ball off for Johnson, and the latter fired home from close range.
Cardiff nearly grabbed their second goal in as many minutes, only for keeper King to deny Harris by clearing with his feet.
The visitors made a double change, and clearly buoyed by their goal, pressed forward with greater urgency in search of a second goal. This duly arrived on 75 minutes courtesy of Johnson, who took advantage of a slip by Sid Nelson to rifle home at the second attempt after his first shot had been blocked.
Millwall responded positively and Ben Thompson was unlucky with a thunderous drive that flashed narrowly wide from 25 yards, but despite a late spell of pressure the young Lions were unable to grab that elusive leveller.
In fact it was Cardiff who so nearly added a third deep into injury-time, but Theo Wharton's shot flew a fraction wide of the target.
Millwall: King, Siafa, Casey, Thompson, Nelson, Wood, Webb, Farrell (Twardek, 61mins), Marquis, Pavey, O'Brien. Subs not used: Girling, Philpot, Rylah, Peter.
Scorer: Marquis (62mins)
Cardiff City: Wilson, Barnum-Bobb, John, James, Wickham, Wharton, Harris, Southam (Roche, 72mins), Johnson, O'Sullivan, Burgstaller (Hill, 72mins). Subs not used: B Watkins, Horton, Bowen.
Scorer: Johnson (63, 75mins)