The 27-year-old handed in a transfer request at Queens Park Rangers in order to force through a switch to the City Ground where he will hope to be a first-team regular after some frustrating times in West London.
Mackie was a popular figure among R's fans but the feeling was that more high-profile players had kept him sidelined too often.
"It's a massive opportunity, a massive club and I'm so excited," he said.
"There are quality players already here and a top manager, who is extremely ambitious. I am massively ambitious too and I cannot wait."
Source: Forest MAD