The teenager, who is aged 17 and so cannot be named, was convicted of the manslaughter of 42-year-old Joseph Cullimore at Southwark Crown Court on Thursday, 2 May and sentenced to six years imprisonment.
Police were called on Friday, 17 August at around 4.40am to an address in Flaxen Road, E4 to report of a stabbing.
Officers attended and found Mr Cullimore suffering from stab injuries. He was pronounced dead at the scene at 5.22am.
A post-mortem examination established his cause of death as a fatal stab wound to the torso.
Mr Cullimore was stabbed by the teenager at the address shortly after he knocked at his door.
The teenager was arrested at the scene and was charged the next day after admitting the brutal assault.
The teenager, who was 16 at the time, knew Mr Cullimore but a motive for the attack has never been established.