Harris, who left Millwall to join Forest in 2004 before returning to the Den in January, headed home the opener for Millwall shortly after the interval to give Forest manager Colin Calderwood a painful reminder of what he is missing.
Harris scored only once during his time at the City Ground, but after scoring his fifth goal since returning to Millwall, the in-form striker is just eight short of Teddy Sheringham's club record haul of 111.
Inspired by the continuing good form of Ryan Smith, the fleet-footed winger on loan from Derby, Millwall began in confident fashion and might have gone ahead as early as the third minute.
In a move identical in all but outcome to the one that created Millwall's opener against Swansea last week, Smith's teasing cross ricocheted off the bar before bouncing to safety.
It was an early warning for Forest, and when David Brammer's solid long-range effort then sent Paul Smith sprawling to his left to palm over, it looked like Calderwood's men could be in for a long afternoon.
But after an eventful opening there was a prolonged lull before the game exploded into life again just after the break.
Smith blasted narrowly wide and Marvin Elliott hit the woodwork with a deflected shot before Hubertz crossed for Harris to head home at the far post.
Kris Commons lead the Forest fightback, firing a quartet of well-struck shots at Lenny Pidgeley's goal, the pick of which beat the Millwall goalkeeper only to cannon off the post.
But despite finishing strongly Forest could not conjure an equaliser and the Lions claimed another notable scalp at The Den.