Richard Mannington Bowes, who received head injuries in the attack, died after being removed from life support, The Daily Telegraph reported Friday.
The suspect was jailed in west London on suspicion of murder, rioting and carrying out three burglaries, Scotland Yard said.
Police described Mannington Bowes as "reclusive and private" and said it took them two days to determine his identity following the Monday attack because he cut himself off from the world since he retired as an accountant.
Neighbors told the Telegraph Mannington Bowes had "old-fashioned values" and regularly chastised youths for littering outside of his apartment building.
Mannington Bowes never regained consciousness after the attack and died shortly before midnight Thursday.
"This was a brutal incident that resulted in the senseless killing of an innocent man," said John McFarlane, detective chief inspector of London Metropolitan Police's Homicide and Serious Crime Command.
Teens arrested in the rioting, looting and arson were being treated as youthful offenders, leaving arraignments with no criminal penalties, officials said.
The prospective judicial sentences sharply differed from Prime Minister David Cameron's promise Thursday of "swift justice" for those who carried out the violence, no matter what their age.
"People taking part in violent disorder should expect to go to prison," he told a House of Commons emergency session to which lawmakers had been summoned from their vacations.
"To the lawless minority, the criminals who've taken what they can get, I say this -- we will track you down, we will find you, we will charge you, we will punish you. You will pay for what you have done," Cameron said.
More than 1,500 arrests have been made since the frenzy of violence broke out Saturday, the BBC reported. Most of those arrested were in London.
Roughly half of the 240 people appearing in London courts through Thursday were under age 18 and, as youthful offenders, were considered by judges to have made youthful mistakes rather than criminal acts, The Daily Telegraph reported.
Conservative Member of Parliament James Clappison said he was "very concerned" about young rioters' light sentences of months, rather than the prospect of serving prison time.
"The courts are failing to hand down sufficiently tough sentences," the Telegraph quoted him as saying. "We need deterrent sentences, but we are not seeing that. Magistrates need to think again."
Justice Secretary Kenneth Clarke backed the courts and rejected calls for new sentencing rules.
"Enough" prison sentences are being imposed, he said in a BBC radio interview.
"The system is working," he said.