Pieter Doorewaard, 28, and Philip Schutte, 35, were found to have killed 15-year-old Matlhomola Mosweu on April 20, 2017, after claiming they caught him taking a plant from a farm in the area, Presstv Reported.
Doorewaard was jailed for at least 18 years and Schutte was jailed for at least 23.
The boy died after being thrown out of a moving vehicle driven by the pair and suffering a broken neck, in a case that sparked rioting and looting of white-owned businesses in the town of Coligny.
The men had claimed that the teen jumped off the truck as they drove him to the police but it was Schutte who was found to have thrown the boy to his death.
Judge Ronald Hendricks previously also found the pair guilty of kidnapping and intimidation.
"Murder is undoubtedly the most serious offence that can be committed," said Hendricks.
"You picked up the deceased and threw him from the van onto the ground.”
"Your actions that day... were indeed disgraceful."
The judge paused as his comments were translated into Afrikaans, the language often spoken by white farmers in South Africa.
"It cannot be ignored that the community revolted as a result of this incident," added the judge.
"This was largely because of the inaction of the police.
"The community of Coligny was polarized as a result of this incident."
Other family members watched on as the judge announced his ruling -- as did members of the radical Economic Freedom Fighters Party which has vigorously opposed racism against the country's black majority.
Racially charged incidents between white farm owners and managers and poor black farmhands are common in South Africa.
In 2016 two white farmers in eastern Mpumalanga province forced into a coffin a black man they accused of trespassing.
The case sparked outrage after a video of the incident emerged on social media and the two were handed jail terms of 19 and 16 years.
Judge Hendricks said in mitigation of Doorewaard and Schutte's sentences that "there was no direct intention to kill the deceased."
"(You are) first time offenders," he added giving reasons for not imposing life sentences.