With relegation rivals Crystal Palace and Sheffield Wednesday facing each other on the last game of the campaign, the Hornets knew three points would be enough and they were in the mood to put on a match-winning performance from the opening whistle.
They duly made the all-important breakthrough on the half-hour and had the result sewn up ten minutes after the break.
By contrast, the Royals were completely out of sorts, came off second best in all departments and failed to trouble Watford keeper Scott Loach during the entire 90 minutes.
The home side had three decent chances to take the lead before they did make the breakthrough.
John Eustace headed a Henri Lansbury free-kick narrowly wide, Martin Taylor then had a half-volley from the edge of the area that was cleared off the line and then Lansbury shot narrowly wide from another set piece.
But Vicarage Road breathed a huge collective sigh of relief after 30 minutes when a Lee Hodson throw was flicked on at the near post by Heidar Helguson and Danny Graham rose highest to head past the static Adam Federici.
That settled the Hornets down and two minutes after the restart they were effectively able to start celebrating safety.
Shane Long made a hash of dealing with Don Cowie's low cross in from the right flank and Helguson turned around 12-yards out and lashed a left-footed shot past the helpless Reading keeper.
If there was any remaining doubt about the outcome, that ended eight minutes later with a moment the experienced Andy Griffin will not look back on with any fondness.
The full-back was under pressure from Graham in the area, but that was still no excuse for the manner in which he miscontrolled, and the former Carlisle United striker didn't need a second invitation to latch on to the loose ball and slide it past Federici to return to the top of his club's scoring charts.