Former Forest winger Commons sent the Rams through to a last-16 tie at home to Manchester United later this month with a deflected second-half effort.
Derby had seemed down and out when they quickly fell two goals behind. Chris Cohen gave Forest a dream start with a goal after only 60 seconds before Nathan Tyson doubled their lead from the penalty spot on 14 minutes.
Rob Hulse halved Derby’s deficit with a well-placed header as the half-hour mark approached and another header from Paul Green restored parity before Commons struck.
The Arctic conditions did little to detract from the white-hot atmosphere inside the City Ground at kick-off.
And Cohen was soon to raise the noise levels by several decibels.
The midfielder collected a return pass from his own short corner and, after taking a touch to make the shooting angle more favourable, curled a left-foot shot past the unsighted Stephen Bywater from the edge of the area.
Tyson made his penalty by breaking the offside trap and racing goalwards before being tripped by Bywater as he took the ball round the Derby keeper.
The Rams halved their deficit in the 28th minute when Hulse rose unchallenged at the far post to guide Gary Teale’s left-wing cross into the opposite corner.
Derby were in the ascendancy and they equalised on the hour when Green took advantage of some more statuesque Forest defending to head home another Teale cross.
Having had a couple of half-chances, Commons finally found the net in the 73rd minute and it proved decisive.
The Scotland international wrong-footed Wes Morgan and created a yard of space for himself.
His shot clipped Morgan as the Forest defender tried to recover and the ball looped beyond Smith and into the far corner.
NOTTM FOREST: Smith, Chambers, Morgan, Breckin, Wilson, McCleary (Heath, 81), Perch (Byrne, 70), McGugan, Thornhill, Cohen, Tyson (Newbold, 85).
DERBY: Bywater, Connolly, Albrechtsen, McEveley, Nyatanga, Barazite, Green, Savage, Teale, Commons, Hulse (Ellington, 82).
Referee: Chris Foy (Merseyside).