The young Bluebirds went into this match full of confidence on the back of two victories, and ten unbeaten in competitive action and took the lead through in-form forward Rhys Healey.
In a topsy-turvy match, City held strong defensively throughout, pressing their opponents in an attempt to score a crucial second. This goal finally came with eleven minutes remaining, Healey again the man in the right place at the perfect time to score his and Cardiff's second - Healey's fourth goal in the last three competitive U21 matches.
This victory was even more important for Kevin Nicholson's men, with play-off rivals Millwall and Colchester United having met earlier in the day. The Lions' 2-1 victory over the U's means it is likely a two horse race between Cardiff City and Millwall for the final Professional Development League Two South play-off spot - the Bluebirds currently holding a one goal advantage over their rivals, both teams with 50 points.
U21 FINAL SCORE: WATFORD 0-2 CARDIFF CITY (Healey, 12' 79')
Watford: Simpson, Lewis, Johnson, Hoban, Trialist, Murray, Stewart, Charles, Ovenden, Jakubiak, Oulare. Subs: Gartside, Adeyemo, Mensah, Young.
Cardiff City: O'Reilly, Rees, R. Menayese, Blaise, Tutonda, Patten, Southam, Veale, Wharton, Healey, Phipps. Subs: James, Abbruzzese, Humphries, Bird.