Forest boss Karanka, who left Boro in March last year after a dismal run in the Premier League, could do nothing to prevent a defeat which keeps Tony Pulis’ side in the play-off places.
The impressive Daniel Ayala scored a fine opener in the seventh minute and that was followed by Stewart Downing’s low drive in the 31st minute to put Middlesbrough in the driving seat.
Forest offered very little in the final third and never looked like ending a wait for a goal that has extended to five matches.
Boro boss Tony Pulis said: “I was pleased with the first half, I thought we played really well with the movement of the ball and the position we got into and the opportunities and chances.
“But the one disappointing thing was that half-time came, because it stopped the momentum which sometimes happens.
“They had nothing to fear in the second half or worry about and they looked more relaxed on the ball but we still had the best opportunities.
“There are five games to go. We spend far too much time in this country worrying about everyone else and what they are doing instead of concentrating on your own lives and I can only affect what happens here - and that is what I have told the players.
“We just have to focus on ourselves and if it goes for us, brilliant, if it doesn’t, we have to make sure we were properly prepared.
“The important thing is we remain focused and do our best and the players understand that.
“Aitor deserves the plaudits. He did a fantastic job to get them promoted and they showed that respect to him, which is fantastic.”
With nine points separating Forest from the relegation zone beforehand, this was an occasion more about Karanka’s return than anything else for them but he tasted defeat with Downing, a player with whom he failed to gel during his time at Boro, on target.
Even though he kept a low profile on entrance, the Spaniard received a fantastic reception from the Middlesbrough fans when he walked into his technical area for the first time.
By then Middlesbrough - looking to bounce back after picking up just a point over Easter - had already taken the lead courtesy of Ayala’s fifth goal of the season and it was his best one too.
With his back to goal as Forest failed to deal with a corner, the defender brought down the ball, turned and struck a sweet finish inside the top corner from 12 yards.
After that it went quiet until the second goal just after the half-hour mark. Again, like so many times this season for Forest, it was conceded from a corner.
The delivery was not dealt with and though Ayala’s flick-on was helped away by a defender, it only reached the edge of the area where Downing pounced and struck low inside Costel Pantilimon’s right-hand post.
Forest have conceded more goals from corners than any other team in the division this season and that was another two to add to the list; it was almost three soon after when George Friend had a header cleared off the line.
Before half-time Middlesbrough did concede four corners of their own in stoppage-time but showed their opponents how to defend them, with Forest’s best effort a low Lee Tomlin drive saved by Darren Randolph after one of the set-pieces was half-cleared.
After the restart Forest’s defensive weaknesses were there to see again. From another corner Ayala almost had a second when his acrobatic overhead kick flew over the bar.
And even though Forest did gain more of a foothold in the play in the second half they still struggled to cause Randolph problems in the Middlesbrough goal.
Joe Lolley did test the Republic of Ireland goalkeeper from distance but he was on hand to gather and hold comfortably.
The only other chances before the end fell to former Forest man Britt Assombalonga who wasted a couple of great efforts, with Patrick Bamford also missing the target.