Goals from Tommy Smith and Nathan Ellington gave Watford victory and brought Ipswich's 15-game unbeaten home league record to an end.
Smith is congratulated by Henderson and Ellington after scoring Watford's opener
The hosts were the only team in the Football League to still be unbeaten at home prior to kick-off.
But Smith's header from Mathew Sadler's corner put Watford ahead on 33 minutes before Alan Lee hit the bar and then Ellington slotted in to make it 2-0.
Ipswich pulled one back when Jonathan Walters poked in from six yards.
Ipswich boss Jim Magilton:
"The best team won. They started better, dominated us physically and didn't allow us to play.
"Watford are a good side and any team that finishes above them in the table will get promoted.
"The did a very professional job and we didn't have enough courage in our conviction. We didn't have enough people believing we could win."
Watford boss Aidy Boothroyd:
"In terms of winning, this was a 10 out of 10 effort. In terms of style, it was probably a five.
"Ipswich is a tough place to come to. We had to defend for long periods and be resilient and, thankfully, we managed to hang on.
"We scored at the right time and it was important that when we had decent spells in the game we used them well."
Ipswich: Bywater, Wright, De Vos, Bruce, Harding, Norris, Miller (Lee 46), Garvan (Sumulikoski 60), Quinn, Walters, Counago (Haynes 59).
Subs Not Used: Colgan, Naylor.
Goals: Walters 71.
Watford: Lee, Mariappa, Shittu, Bromby, Sadler, Smith (Kabba 82), Bangura, Eustace, Ainsworth (Stewart 54), Darius Henderson, Ellington (O'Toole 77).
Subs Not Used: Poom, Doyley.
Booked: Eustace, Darius Henderson, Bangura, Bromby.
Goals: Smith 33, Ellington 56.
Ref: Andy D'Urso (Essex).