Prosecutors said Ben Hope, 39, and Jason Richards, 38, were paid 1,000 pounds ($1,600) each to kill a man who owed money to an underworld businessman.
But the pair, both heroin addicts, killed 17-year-old Aamir Siddiqi on his doorstep instead. Hope and Richards also attacked the teen's parents as they tried to save their son, who had wanted to be a lawyer.
A jury at Swansea Crown Court unanimously found the pair guilty of Siddiqi's 2010 murder in Cardiff, Wales. The two also were convicted of the attempted murder of Siddiqi's parents.
Prosecutors said the killers, former prison cellmates, had acted with "staggering incompetence."
After carrying out the attack at a house on the wrong street, Hope drove away from the crime in a stolen car. It was found abandoned, covered in his fingerprints and the victim's blood.
Richards denied being at the scene of the murder, but the jury did not believe him.
The murdered teen's sister, Umbareen Siddiqi, said Aamir had been "the heartbeat of our family."
"On the 11th of April 2010, a house which was previously filled with love and laughter was brutally destroyed by the callous, vicious attack on our parents and our brother," she said outside court. "Within seconds our lives changed forever.
"Aamir was a beautiful person with a bright future. If he was still with us he would be looking forward to turning 21 this year and completing his law degree."
Judge John Royce said a life term in prison was inevitable when the two men are sentenced Feb. 8.