The win cements Brentford's third-place finish in the Sky Bet Championship this season, making sure of a play-off Semi-Final against whichever team finishes sixth from AFC Bournemouth, Seansea City or Barnsley.
Fresh off his first goal for Brentford B, and just three weeks since his First Team debut, Mads was handed his first start against Watford. The Hornets secured promotion automatically with a win over Millwall last weekend.
Marcus Forss opened the scoring just 55 seconds after half-time before Ivan Toney netted a penalty to score his record-equalling 30th goal of the season. His two-step spot-kick saw him draw level with Glenn Murray, who set the record for league goals at Crystal Palace in 2013.
Brentford play their final game of the regular season next Saturday at Ashton Gate, Bristol City hosting the play-off-bound Bees on Saturday 8 May.