Darius Henderson bagged his fourth and fifth goals in three games to send Watford to the top of the Championship.
Watford's Darius Henderson continued his hot scoring streak
The striker, preferred to Marlon King, opened the Hornets' account at Ninian Park in the 17th minute by scrambling home Jay DeMerit's pass from close in.
Jimmy Floyd Hasselbaink brought Cardiff level after the break with his first goal for the club, hitting a powerful 20-yard drive into the bottom corner.
But Henderson struck again from close range, turning in Gavin Mahon's cross.
Cardiff manager Dave Jones:
"You cannot make mistakes at the back like that against a team as good as Watford.
"We never lost the game because we were outplayed, but because we switched off.
"We have to concentrate, particularly on set plays, as we actually cleared the ball before both goals but were not awake for the second phase."
Watford manager Adrian Boothroyd:
"We weathered some storms, defended well and ground out a result without playing that well.
"It says a lot that (goalkeeper) Mart Poom was our man-of-the-match with some great saves.
"Darius Henderson is on fire, scoring goals and I've not been in the position before to be able to leave Marlon King and Jobi McAnuff on the bench."
Cardiff: Turnbull, Gunter (Thompson 82), McNaughton, Johnson, Capaldi, Sinclair, McPhail, Rae, Parry, Fowler (Whittingham 84), Hasselbaink. Subs Not Used: Oakes, Ledley, Blake.
Goals: Hasselbaink 61.
Watford: Poom, Doyley, Shittu, DeMerit, Mariappa (O'Toole 73), Stewart, Mahon, Johnson (McAnuff 79), Smith, Darius Henderson, Ellington (King 59). Subs Not Used: Lee, Priskin.
Goals: Darius Henderson 17, 78.
Ref: Lee Probert (Wiltshire).