The competition, now known as the Carling Cup after numerous changes of name, could pave the way for an historic five-trophy season for the Red Devils.
But the world, European and English champions were put through the full gamut of emotions as Championship side Derby staged an heroic late fightback.
Leading 1-0 from the first leg at Pride Park, Derby scored twice in a dramatic last 10 minutes which saw three goals.
It all seemed to be going according to plan for United at half time when they led 3-0.
Sir Alex Ferguson's side were on level terms over the two legs as early as the 16th minute thanks to superb goal by Luis Nani.
The Portuguese winger picked the ball up 35 yards out on the left, cut inside Gary Teale and hit a stunning left-foot shot into the top left corner of the net - stinging goalkeeper Roy Carroll's fingers on the way.
United went 2-0 in front six minutes later with the move started and finished by John O'Shea.
The Republic of Ireland defender rolled the ball forward for Carlos Tevez on the last edge of the Derby box.
Tevez turned the ball inside to Danny Welbeck who clipped it forward for the oncoming O'Shea to stroke the ball home right footed from 12 yards.
The game looked over 11 minutes later when Rafael Da Silva's perfectly measured cross from the right corner of the Derby penalty area was headed in by the unmarked Tevez on the edge of the six-yard box.
Rafael came close with a powerful shot soon after, but limped off two minutes before the interval with a hamstring injury and was replaced by Darren Fletcher.
United looked in little trouble as Ferguson brought on Cristiano Ronaldo for Ryan Giggs and 19-year-old debutant James Chester for Gary Neville as the game tipped to the midway spot of the second half.
But the use of all his substitutes almost came back to haunt the United manager.
Central defender Jonny Evans suffered a recurrence of an ankle injury and Derby looked a greater threat when Giles Barnes replaced Kris Commons in the 68th minute.
Eleven minutes later, the limping Evans brought down Paul Green in the United box and referee Mike Dean pointed to the penalty spot.
Up stepped Barnes to drive his penalty right footed into the top right corner of the net as United goalkeeper Ben Foster guessed correctly but was beaten by the power.
It looked all over when a glorious United move involving Ronaldo and Nani ended with Tevez being brought down by Carroll as he tried to round the Derby goalkeeper inside the box.
Ronaldo drove his ensuing spot-kick right footed hard and low into the bottom right corner of the net with Carroll almost getting to the ball.
But Derby were not finished as Barnes scored with a brilliant 30-yard free-kick from the left side of the United box into the bottom right corner of the net off the foot of the post.
United midfielder Anderson also suffered a late ankle injury to leave the Red Devils counting the cost of another final place.