Samuel Geary was 16 years old when he and Reginald Dobbins went to the home of Keith Suitter, 61, with a plan to purchase drugs.
Geary claimed Dobbins began beating Suitter with a hammer and then stabbed him multiple times when he couldn't do it himself.
Both were charged with murder following the 2015 attack and Geary eventually pleaded guilty.
He was ordered today to spend 40 years in prison.
Dobbins was found guilty of murder following his trial in June and is still awaiting his sentence.