Cardiff have not lost to Forest in 10 meetings and they have won nine of those games after Nathaniel Mendez-Laing's precise finish earned them all three points at the City Ground.
Harris, who endured a tough spell as a striker while playing for Forest, would have appreciated the quality of the game's only goal, as the Cardiff man found the bottom corner of the net from the edge of the box to help the newly appointed manager to another victory in his third game in charge.
But Forest's lack of finishing power was also decisive as they registered 31 efforts on goal but only hit the target twice.
Forest carved out the first meaningful opportunity as Joe Lolley and Lewis Grabban combined to find Sammy Ameobi in space in the box, but the winger lost his footing as he hit his shot, which bobbled harmlessly wide.
The home side had been dominant in the opening exchanges, but it was Cardiff who took the lead in the 14th minute, with their first effort.
After he was fed the ball by Lee Tomlin, Mendez-Laing was given the time and space to curl a precise, curling shot into the bottom corner, beyond the reach of Bryce Samba, to deny him what would have been a fourth consecutive clean sheet.
Forest looked to respond as Ameobi drove a wild shot high and wide, but Samba had to save when Marlon Pack looked to catch him out with a shot from a free-kick, when it looked as though he was going to cross.
Lolley fired wide and Ameobi wildly over as Forest continued to look for an opening, while the latter and Carvalho came a little closer with subsequent efforts, but still without hitting the target.
Forest were having plenty of attempts, but Ameobi was denied by a simple save from Neil Etheridge and Cash was off target again.
The second half saw Jack Robinson's curling free-kick kept out by Etheridge, but Worrall had to clear off the line to deny Cardiff a second goal as Junior Hoilett was picked out by Gary Madine only a few yards out.
Ben Watson and substitute Alfa Semedo both cleared the bar as Forest continued to create – and waste – a series of half chances. Forest notched up their 20th effort on goal with 20 minutes still to play, but only one had been on target.
Forest defender Robinson come agonisingly close with a curling shot, which missed the target by inches.
Cash finally forced another save from Etheridge with a well-struck shot, but it was too little, too late for Forest.