The Bees opened the scoring in the 57th minute when Forss fired home from 12 yards after being fed by Toney following Bryan Mbeumo's cross.
Toney then notched for the 13th time this season with a penalty eight minutes from time after Rotherham substitute Mickel Miller was sent off for hauling down Sergi Canos.
It was a third successive win for Thomas Frank's side, who are now unbeaten in eight as they continue to shake off the disappointment of last season's play-off final heartache.
Rotherham, who held Bournemouth to a 2-2 draw at the New York Stadium on Saturday, were competitive in the first half but never threatened a fightback after Brentford broke the deadlock with the defeat extending their winless run to four games.