Troy Deeney put Watford in the lead at the Amex Stadium just before the half-hour mark, and Matej Vydra secured the points with his side’s second in the final minutes.
With promotion rivals Middlesbrough and Norwich both dropping points later in the day, Watford are now certain to finish in the automatic promotion spots.
After it was confirmed that Slavisa Jokanovic’s side will feature in the top flight of English football next season, Assombalonga posted the following message on Twitter for his former teammate Deeney.
Well done to Watford on automatic @T_Deeney I see you brother— Britt Assombalonga (@BrittOfficials) April 25, 2015
Forest were also in action today, but they failed to win, going down 2-1 to Ipswich Town away from home at Portman Road.
The Nottingham-based outfit are 12th in the Championship table at the moment with 59 points from 45 matches, just a point behind 10th-placed Charlton Athletic.