The youth stood crying in the dock at Inner London Crown Court as a "horrified and sad" Judge Quentin Campbell said he had no option but to lock him up.
He was convicted last September of possessing heroin and crack cocaine with intent to supply.
"Crack cocaine and heroin are the two most common ... and most dangerous class A drugs that are available," he said.
While he accepted the youngster was not the "prime mover" in the drug dealing, there was no doubt he was a "perfectly willing participant".
The court heard how the youngster - acquitted at another court recently of two counts of robbery - was spotted cycling with a 15-year-old youth by four plain clothes police officers in south London.
Drugs stash found
He tried to run away but was caught and found with 18 rocks of crack cocaine and two wraps of heroin worth £500 in his trouser pocket.
Beneath them was £400 cash.
More crack cocaine, a small amount of cannabis and a further £1,600 was found shortly afterwards at a friend's flat, which the boy had moved into after a row with his mother.
The 15-year-old was also arrested, but was not charged through lack of evidence.
The judge warned the boy, who has dropped out of school, that he would have faced five years' imprisonment had he been an adult.
Youngster distressed
"I make no further comment as to whether anybody is to blame that no educational authority took up the fact that you were not effectively going to school at all."
As the door leading to the cells was opened, the youngster turned towards his mother and began screaming that he wanted to see her.
Detective Constable Ian Dickson, said the case saddened the officers working on it.
None of them had ever heard of anyone so young being convicted of dealing in Class A drugs, he added.
After the verdict the youngster's mother, 41, said that while she felt "terrible", nothing about her son's behaviour aroused any suspicions.
She insisted he was a "happy-go-lucky" individual who simply got mixed up with "unsavoury underworld types" at a club and was "led astray".