John Mackey, 87, was allegedly targeted by unemployed Peter Augustine after he visited a Co-op store in Manor House on the afternoon of May 6.
Passers-by allegedly saw the 59-year-old defendant attacking Mr Mackey and searching his jacket as he lay on the ground, the Old Bailey was told.
Prosecutor Jane Bickerstaff KC said another member of the public alerted police after she heard a shout of “give me the bag” and saw Mr Mackey lying inert on the pavement
Mr Mackey, who was bleeding from his head, regained consciousness but was unable to tell officers what had happened, she said.
He was treated in hospital for bleeding on the brain and broken ribs, but died from his injuries two days later.
Ms Bickerstaff said police pieced together events from CCTV footage, although attack itself was not caught on camera.
Mr Mackey had set off from his address shortly after 5pm wearing a distinctive trilby hat, grey mackintosh coat and carrying a walking stick and black bag.
At 5.12pm, he went into the Co-op where he bought cornflakes, an own brand cottage pie and Mirror newspaper which he put in his bag.
He paid for his goods using a £20 note and put the change of £11.50 in his trouser pocket.
Eighteen minutes later, his path allegedly crossed with Augustine as he crossed the road and the defendant began to follow him.
Mr Mackey then went into Manor Kebab at 5.36pm and bought sausage and chips as the defendant allegedly loitered outside.
The victim was last seen on camera at 5.50pm and Augustine reappeared on CCTV three minutes later carrying what looked like the victim’s bag, jurors heard.
A later search of the defendant’s room uncovered what was left of Mr Mackey’s groceries and wrapping from the cottage pie, and sausage and chips, it was alleged.
Following his arrest, Augustine said in police interview that he saw Mr Mackey on the floor, tapped him lightly on his leg and said, “alright guv”.
He said that he thought that Mr Mackey was drunk and walked off, taking with him a black bag he found on the road.
Augustine told officers: “The wind must have blew it on the side, so I picked it up and I looked inside (sic).”
He said that it only contained a newspaper and claimed not to have noticed Mr Mackey before finding him on the ground.
Ms Bickerstaff told jurors that Mr Mackey was 5ft 5in tall and weighed less than nine stone.
She said: “He would have been largely unable to put up any resistance to the attack due to his age and infirmity.”
The prosecutor told jurors that at the time, Augustine was living in “reduced circumstances” and was effectively homeless.
She said he had used “extreme force” after he targeted Mr Mackey.
Ms Bickerstaff told jurors: “When the defendant kicked and punched an elderly, vulnerable and largely defenceless man, who was prone on the floor, the Crown says that he intended to cause him really serious harm.”
Augustine has denied robbery and murder. The Old Bailey trial continues.
Detective Chief Inspector Mark Rogers, previously said: “John was a proud Irishman and an innocent member of the public, and was walking back home after running errands when he was the victim of this tragic incident.
“His family tell us he was one of 11 children. He was born in Callan, County Kilkenny, Ireland and moved to London in the 1950s.
“He was uncle to a remarkable 22 nieces and nephews and a brother to his remaining four siblings, they will all miss him.
“John’s family describe him as funny and charming. He was a life-long Arsenal fan, who was adored by his family and was loved by his local community.”
