Goals from Adel Taarabt and Tommy Smith secured victory at Vicarage Road on Saturday, meaning Rangers cannot be caught by third-placed Cardiff and should have booked their place in the top flight as well as clinching the league crown.
Neil Warnock and his coaching staff all embraced on the touchline, but the celebrations will be slightly muted with the club now nervously awaiting the outcome of next week's FA hearing into the transfer of Argentinian midfielder Alejandro Faurlin.
Their fans shrugged off fears of a points deduction and hailed their side as 'champions', but Watford's fans responded by warning them 'you're gonna lose in the play-offs'.
The wheels fell off Rangers' promotion push at Vicarage Road last season courtesy of a 3-1 defeat and a dressing-room bust-up which led to the departure of Jim Magilton as manager.
And their cause was not helped this time around when goalkeeper Paddy Kenny was injured in the warm-up, while Fitz Hall's hamstring lasted only 20 minutes longer.
However, Taarabt's 19th goal of the season and Smith's injury-time strike means only the FA can deny them top-flight football for the first time in 15 years.
Hornets defender Lloyd Doyley scored his first, and to date only, career goal in last year's encounter, and he immediately attempted to repeat the trick with a neat turn inside and curling shot which flew just wide.
Rangers made a lively start, though, with ex-Hornet Heidar Helguson firing wide and heading straight at Scott Loach in the opening stages.
Loach then spilled Taarabt's free-kick but his blushes were spared by Adrian Mariappa, before Faurlin blazed a shot over.
Watford should have taken the lead on the half-hour but top scorer Danny Graham somehow headed Troy Deeney's cross wide from all of three yards out.
The visitors thought they had taken the lead but after 40 minutes but Bradley Orr was offside when he tapped in Taarabt's cross, and Rangers' stand-in keeper Radek Cerny saved well from Martin Taylor's header before the interval.
Faurlin, of all people, almost came up with a stunning goal early in the second half but his first-time shot from the edge of the area was superbly tipped away by Loach.
Faurlin was having an impact at both ends and he blocked a goalbound effort from Deeney from a corner early in the second half.
But the goal Rangers had been threatening finally came in the 76th minute when Smith charged down the left and crossed for Taarabt to sidefoot past Loach at the near post.
It was a fitting strike as Taarabt's goals, tricks - and tantrums - have illuminated Rangers' promotion push this season.
Smith ran through and tucked in the second in stoppage time to spark wild scenes in the away end, and Warnock and Co will now wait and see if the celebrations are premature.