Forest threw away leads in their previous two Championship matches against Blackburn and Millwall, having to settle for a 2-2 draw on both occasions.
However, Karanka’s team won at a canter at the Riverside, despite Jack Robinson being red-carded in the closing stages for a second bookable offence.
Joe Lolley and Lewis Grabban scored to lift Forest up to fifth and Karanka said: “In the last week we were winning at Blackburn and against Millwall and we couldn’t manage to win those games.
“Today has been perfect. We were winning, we managed the game even when we had one player less.
“I am really pleased for the players because we are now in the position we deserve to be.
“We are fifth. The position in the table is a reflection of the team and now we are there because we deserve to be. The team is improving, the team is growing and that is the main thing.”
Karanka left Middlesbrough in March 2017 after a three-year tenure during which time he achieved a promotion and suffered relegation from the Premier League.
He added: “This club is and will always be special for me, it was always be in my heart, but when I come here as an opponent it is only three points.
“It will always be special for me. It is true that the first return was incredible. I had a good experience with everyone in the stadium.
“Every time I come back here will be special.
“They have everything you need. They have a very good squad, a very good manager and a very good crowd.”
Middlesbrough were flat, putting in a below-par performance and conceded at home for the first time this season.
Manager Tony Pulis admitted: “I don’t think anybody saw that performance coming, but that’s the way the league is, we’re not going to be perfect every week and it brings everybody down to earth in respect of the expectations.
“It’s a tough league and everybody in every game turns up.
“We need to be much more clinical and we need to do something in that respect in the final third.”
Boro were on the back foot from the off, though Forest did not score until early in the second half, Lolley firing home after Daniel Ayala had been turned in the centre of the pitch by Daryl Murphy.
At 1-0 Pulis made a triple substitution with Britt Assombaloga among those sent on, but the former Forest striker missed a great chance.
Pulis added: “I thought we started really, really poorly, the first 20 minutes we were like a bag of potatoes, we were so sloppy and so on the back foot, we never, never got going.
“The goal was a dreadful goal and very unlike us. Then you need to score, you need to get back into the game as quickly as you can and the big, big point, the turning point was Britt’s chance.
“I think if Britt scores then on, what was it, 60-odd minutes? Then it’s a different atmosphere and a different approach from the team.”