Championship wrap: Watford hit Leeds for six, Palace stay top
Gianfranco Zola's men, who had not beaten Leeds since May 2006, hit the front after 28 minutes courtesy of Matej Vydra.
Leeds' woes were compounded before half-time when defender Jason Pearce received his marching orders for a rash challenge on Vydra.
From that point the visitors dominated and made the most of their numerical advantage with a five-goal second half.
Almen Abdi, who had to be stretchered off later in the game, opened up a two-goal lead for Watford just after the hour-mark before Mark Yeates added a third with 15 minutes remaining at Elland Road.
Former Stoke and Sheffield United midfielder Michael Tonge pulled a goal back for the hosts from the penalty spot but it would be in vain as Watford added three late goals through Vydra, Sean Murray and Troy Deeney.
The result moves Watford up to 12th, four points adrift of the playoff places, while Leeds drops to 17th as a result of their six-game winless streak.
Crystal Palace came from behind to record a 2-1 win at Peterborough and extend their unbeaten run to 13 matches.
Grant McCann had put the hosts ahead after six minutes but two goals in the final minutes from Andre Mortiz and Kagisho Dikgacoi handed Ian Holloway his first away victory as manager.
Palace are a solitary point ahead of Middlesbrough, who accounted for Sheffield United on Friday.
Ipswich Town notched up their second win in three league games with a hard-fought 2-1 win at home to Burnley.
Former Sunderland striker Daryl Murphy opened the scoring six minutes into the second half before Burnley substitute Sam Vokes levelled proceedings.
However, DJ Campbell, on loan from Premier League side Queens Park Rangers, secured Ipswich's first home win in 11 attempts with an 87th-minute winner.
Elsewhere on Saturday, Cardiff City returned to winning ways with a 2-1 victory over fellow promotion hopefuls Hull City, Wolverhampton and Brighton shared the spoils in a thrilling 3-3 draw at Molineux Stadium, and in-demand winger Thomas Ince scored his third goal in three games as Blackpool drew 2-2 at home to Bolton.
Millwall made it nine games unbeaten following their 2-1 triumph against Derby County, Jermaine Beckford's 36th-minute strike proved the difference as Huddersfield edged Barsnley, while Blackburn and Birmingham played out a goalless draw.