Moses Mathias, who is serving a life sentence for killing 16-year-old Giuseppe Gregory when he was just 15, bragged of his "lazy" life, designer wardrobe and television watching schedule.
According to the Sun newspaper, he used his Facebook page to showcase his expensive shoes, acupuncture treatments and muscular figure built up from hours at the prison gym.
It is understood the profile, which has now been taken down, was updated using a contraband mobile phone smuggled into the prison.
In one post which is sure to infuriate law-abiding citizens, Mathias wrote about his daily schedule at HMP Alcourse, Merseyside.
The gunman, who is now 18, is reported to have said: "Havin lunch and gettin bak in bed. am havin a lazy one, just waitin 4 the voice 2nyt cant wait!"
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He also published photographs of his possessions, including a line of expensive trainers.
He showed pictures of Dolce & Gabbana shoes, a Playboy branded t-shirt and a G-Shock watch, the newspaper reported, writing on one occasion: "Wardrobe looking very Italian today."
He also told friends: "Yh i have a tv, 20channels it is gd x"
In 2009, police named Mathias as the killer of Giuseppe Gregory, after he was shot in the head while sitting in a car, outside a pub in Stretford, Greater Manchester.
At the time, enormous wanted posters were beamed across the city and a £15,000 reward offered in an attempt to catch him, as the public were warned he was dangerous.
He was later caught in Amsterdam and jailed for a minimum of 18 years.
After sentencing, he was revealed to have idolised gangland life and was said to have an "obsession" with guns and respect.
In court, Judge Tim Holroyde told him: ''You wholly embraced gang culture from an early age and as a 13 year old you claimed gang membership. You seem to glorify gang culture."
A relative of Guiseppe's mother told the Sun today: "She's disgusted that Mathias can get a mobile phone in jail.
"He has shown no remorse."
The newspaper claims the killer had been put in solitary confinement and his cell searched after the Ministry of Justice were made aware of the Facebook page, which had been set up under a false name,
The Prison Service confirmed inmates do not have official access to the internet and are banned from updating their Facebook profiles.