Forest on Friday wrapped up the signature of the QPR forward on a three-year deal for an undisclosed fee.
The 27-year-old made 29 appearances for the west London club last season but scored just two goals as QPR were relegated to the Championship.
He requested to leave earlier this week in order to find more regular football, having found himself in and out of the side following Harry Redknapp's arrival at Loftus Road in November.
And Mackie is elated with his move to the City Ground, while also admitting he still has "great memories of QPR" having had a "fantastic time" with them.
He told Sky Sports News Radio: "I'm absolutely delighted to get the move to Nottingham Forest. It's a massive football club and I can't wait to get playing now.
"There was speculation and it was just down to the clubs in the last week or so getting down to the finer details and they ended up getting it done and I'm more than happy.
"Of course it's a hard one but you have to be professional about it and the main thing is when you go to a new club you need to be as fit as possible.
"I haven't played the games in pre-season but I feel fit, I just need that little bit of sharpness now so I'm glad it has been pushed over the line. I've got a week to train before the start of the season to try and get a bit of sharpness."
Mackie admits the presence of Billy Davies in the Forest hot-seat was a big lure for him, saying: "The manager obviously was a big pull for me but the club as a whole sells itself.
"It's massive, it's got a great fanbase, fantastic facilities and in my opinion is all geared up to be right up there this season."
He added: "I've got great memories of QPR. I had a fantastic time and got promoted with them, enjoying some unbelievable memories which will always stay with me.
"I made some fantastic friends there and I'm full of respect for the owners, the manager and all of the staff they've got there so I really hope they have a great season as well and can find themselves back in the Premier League.
"I had a great time there but now I'm fully focused on obviously helping Nottingham Forest and doing as much as I can to be part of it for us to push for promotion."