Mackie, who Reds boss Billy Davies tried to sign during his first spell in charge at the City Ground when the forward was a Plymouth player, has put pen to paper on a three-year deal.
The 27-year-old was deemed surplus to requirements last season as QPR were relegated from the Premier League and with his situation at the west London club showing little sign of improvement he submitted a transfer request on Thursday, which was "reluctantly accepted" by Rangers.
Forest had already had a bid for the player turned down but when they returned with a second offer, believed to be worth in the region of £1million, the Hoops agreed to sell.
Mackie played a key role as QPR won promotion to the Premier League in 2011, scoring nine goals during the first half of the season before suffering a broken leg. He made 85 appearances in total for Rangers, scoring 18 goals
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- Premier League