United retained the Shield after being taken to a penalty shoot-out for the second year. Portsmouth displayed very good performance in the 90 minutes but failed to keep up in the penalty shootout.
Lassana Diarra and Glen Johnson of Portsmouth missed their efforts over the bar while substitute Arnold Mvuemba's effort was blocked by Edwin van der Sar.
Carlos Tevez, Ryan Giggs and Michael Carrick all scored forManU.
United, who beat Chelsea in a shoot-out last year, were worthy winners despite fielding a side lacking Wayne Rooney, Cristiano Ronaldo and several other luminaries.