Iulian Constantin Anisiu, 22ys, of no fixed address, was sentenced to six years imprisonment for robbery and possession of an imitation firearm at Wood Green Crown Court on Tuesday, 12 October.
Anisiu had pleaded guilty to the above offences on 4 October.
Detective Sergeant Adam Rodger, from the MPS Finchley Flying Squad, said:
"This was an appalling crime where a frail and elderly man was beaten up so badly, he ended up in intensive care with substantial internal injuries. The victim now requires specialist care at home and has flashbacks from the attack.
"The MPS Flying Squad is committed to detecting crimes and apprehending violent criminals to make our business communities feel safer."
Officers were called at approximately 13:10hrs on 7 July, to reports of an armed robbery at a Post Office in East Barnet Road, junction with Churchill Road, Barnet.
Anisiu, who was with two white male suspects (no further descriptions), parked a green Ford Ka outside the venue before entering the shop. Two of the men were in possession of black imitation firearms.
Two unidentified suspects forced their way to the back office and in the process they pushed a 74 year-old shop manager down to the ground. He was repeatedly kicked in the stomach. The injured man was taken to hospital in a serious condition, where he underwent treatment for abdominal injuries. He remained in intensive care for four days.
All three suspects then made off in the Ford Ka, along Churchill Road in the direction of Brunswick Park.
An imitation firearm was recovered at the scene.
Cash and stamps to the approximate value of £11,700 were stolen.
Anisiu was arrested on 30 July in Edgware on suspicion of robbery.