Dean Gabay was just 18 when he killed beggar Janusz Krason with a single punch after being asked for a pound.
The teenager floored the 36-year-old Polish national in the street and then boasted: 'One shot and that's it.'
Mr Krason cracked his head on the pavement and died later the same day after suffering a fractured skull and brain damage.
At the time of the killing on May 31 last year, Gabay was serving a community supervision order for an attack on two shopkeepers.
He was charged with murder but pleaded guilty to manslaughter at the Old Bailey and was jailed for 21 months.
Judge Martin Stephens QC told him: 'In the early hours of the morning a pathetic drink-sodden Polish man approached you, doing no more than ask you for a pound.
'Your response was to punch him in the face so he fell to the ground, fracturing his skull.
'You were in something of an aggressive mood and you lashed out with terrible results.
'Of course you did not intent to cause serious injury but gratuitous violence such as this on the streets of London cannot be tolerated.
'You have recent previous convictions for gratuitous violence. I cannot avoid passing a custodial sentence.'
The court heard Mr Krason (pictured right) was a heavy drinker of alcohol and lived at a disused garage in Birchen Grove, Neasden, northwest London.
At around 4am on May 31, 2008, he approached a group of youths in Neasden Lane North and asked for money.
'The man asked him if he was a Polish Catholic,' said prosecutor Anthony Leonard QC.
'He said ''I'm a protestant from Northern Ireland'' and then punched him.
‘The man then stole the victim's phone. He seemed proud of his punch and said ''One shot and that's it''.
The victim’s friends took him back to the garage.
Later in the day they attempted to persuade him to get up but he refused and said he wanted to sleep some more.
When they returned to the garage that, evening Mr Krason’s friends realised was not snoring, and wen they checked for vital signs they could not feel a pulse.
Gabay was arrested on June 7 after police traced the stolen mobile phone to the block of flats where he lived.
He told police: 'All I can tell you is he came towards me. I felt intimidated. I only hit him once.
'The man started to ask for money. I said go home at which point the man became aggressive. He was throwing his arms around. I felt intimidated and punched him once.'
The court heard that in November 26, 2007, Gabay was given a 12 month supervision order with 75 hours unpaid work for destroying or damaging property and two counts of assault occasioning actual bodily harm after he was paid to attack two shopkeepers.