The highly-rated midfielder struck in the 23rd minute with a superb 25-yard strike as the Bluebirds slipped to their second home defeat of the campaign.
Substitute Dexter Blackstock added a killer second goal for Billy Davies' men in the 84th minute, tucking the ball home from a fine pass from the influential McGugan.
Yet Blackstock succumbed to a serious injury in the closing stages and was replaced by Nialle Rodney.
It was only Forest's second away win of the season but further underlined their promotion pedigree after last season's play-off semi-final defeat to Blackpool.
Cardiff named Jay Bothroyd in their starting line-up following his England debut as a substitute in Wednesday's 2-1 friendly defeat to France.
They went close in the fourth minute when Craig Bellamy collected possession and hit a 25-yard shot over the crossbar.
Wes Morgan then fouled Seyi Olofinjana and and Peter Whittingham curled a 20-yard free-kick over the crossbar.
Yet Forest were not without a threat themselves and they went close in the 12th minute when Chris Cohen drove a shot just wide from 25 yards.
Seven minutes later, Bothroyd collected possession and drilled a shot just wide from 12 yards out.
Forest then struck the opening blow out of the blue midway through the opening period when McGugan collected a pass from Dele Adebola and drilled a superb 25-yard shot high into the net.
The Nottingham-born midfielder, 22, tried his luck again three minutes later when he shot just wide of the post from the edge of the 18-yard box.
McGugan's goal seemed to stun Cardiff as Forest began to build up a head of steam as the interval approached.
In the 29th minute, former Bluebirds favourite Robert Earnshaw curled a delightful left-footed effort towards goal but Tom Heaton made a routine save.
Forest midfielder Guy Moussi then drilled a shot wide from before Whittingham rifled an effort over the bar for Cardiff.
The Bluebirds made a tactical switch at half-time, replacing the ineffective Michael Chopra with Daniel Drinkwater.
Yet Forest could have been 2-0 ahead eight minutes after the restart when an inswinging corner from Cohen was greeted by Moussi's header from six yards.
The ball flew over the crossbar but it summed up Forest's goal threat as Cardiff tried to haul themselves back into the game.
Adebola then had a shot saved by Heaton before Forest wrapped up the points with six minutes remaining.
McGugan showed intelligence to find Blackstock with a neat pass and the substitute striker, who had replaced Earnshaw in the 70th minute, fired home left-footed from 12 yards before succumbing to a serious-looking injury.
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