Pieter Doorewaard, 28, and Philip Schutte, 35, were found guilty of killing 15-year-old Matlhomola Mosweu in April 2017 after claiming they caught him taking a plant from a farm in the area.
On Wednesday, Doorewaard was sentenced to at least 18 years in prison while Schutte was given a 23-year term.
Judge Ronald 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 said, giving reasons for not imposing life sentences. "However, the aggravating circumstances of this case far outweigh mitigating [factors]."
Mosweu suffered a broken neck and died after being thrown out of a moving vehicle driven by the pair in a case that sparked rioting and looting of white-owned businesses in the town of Coligny.
The men had claimed the teen jumped off the truck as they drove him to the police, but it was found that Schutte threw Mosweu to his death.
Judge Hendricks previously found the pair guilty of kidnapping and intimidation.
"Murder is undoubtedly the most serious offence that can be committed," he said. "You picked up the deceased and threw him from the van onto the ground. Your actions that day were indeed disgraceful."
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; the pair were handed jail terms of between 16 and 19 years.