Tim Norman, the son of Miss Robbie, was convicted of planning the murder of his nephew.
A jury took three days to deliberate. Norman, who already has a history of arrests, will almost certainly spend the rest of his life behind bars.
According to investigators, Norman planned the attack to profit from $450,000 worth of life insurance policies he had taken out on behalf of his nephew and named him as the only beneficiary.
There are rumours that Norman also planned a break-in at his mother's house, blaming his nephew for it.
Terica Ellis, a former exotic dancer, is one of the two conspirators who, according to authorities, helped the former reality star lure Montgomery Jr. to the spot where he was executed.
According to St. Louis Today, the evidence produced at the trial revealed that Norman paid Ellis $10,000 to find the whereabouts of his nephew.
After Travell Anthony Hill killed Montgomery, he later paid $5,000 to Hill using a buddy. Hill also entered a guilty plea.
After the verdict was pronounced, Norman's mother, Sweetie Pie's leading lady Miss Robbie, reportedly cried while her son reportedly sat expressionless. Throughout the trial, she gave Norman her full public support.