First-half headers from captain Richard Keogh and on-loan striker Chris Martin saw Derby end their Leicester hoodoo with victory at Pride Park.
After five successive Championship defeats against the Foxes, Nigel Clough's side broke the sequence with a vital three points to move them clear of the relegation zone.
Substitute Jeff Schlupp pulled a goal back for the visitors just after the hour mark but Leicester could not rescue anything from the game as their winless league run reached five matches.
Leicester manager Nigel Pearson made one change to his side, recalling fit-again left-back Paul Konchesky in place of striker Schlupp.
Derby struck first as Leicester captain Wes Morgan conceded a free-kick wide on the left and Keogh glanced home Jamie Ward's delivery from six yards out.
Leicester almost equalised as Jeff Hendrick lost possession to opposite number Richie Wellens on the edge of his own box but Lloyd Dyer scuffed his volley after being picked out at the far post by David Nugent, who was denied by Frank Fielding shortly afterwards when he let fly from long range.
The Rams finished the first half on top and almost extended their lead when Ward escaped down the left before setting up Martin, but the man on loan from Norwich saw his shot saved by Kasper Schmeichel.
Martin would not have to wait long for a goal, however, as he headed home Forsyth's left-wing cross a minute before the half-time whistle.
Schlupp, who in Konchesky's absence had been filling in as a makeshift left-back, was introduced at the start of the second half with Wellens making way.
And the 20-year-old, who recently spent a month on trial at Manchester United, got Leicester back into the game after 62 minutes with an arrowing left-footed strike across Fielding after being teed up by Nugent's lay-off.
Leicester pushed for an equaliser but never really tested Fielding as Derby held on for a first Championship victory in nine outings to climb into the top half of the table.
Leicester had been pushed down to sixth with Nottingham Forest's win at Hull earlier in the day and Pearson's team remain in the final play-off spot.