The group was in New York City on Wednesday to announce the formation of Reform Alliance, which will lobby for changes to probation and parole laws, among others. Rubin said the group will commit $50 million toward its efforts, according to Deepti Hajela of the Associated Press.
Kraft described the current system as "not good for America."
"We can make America better if we really cure this problem," he said.
Reform Alliance will be led by CNN host and activist Van Jones, while Rubin will serve as co-chairman alongside Mill. Brooklyn Nets co-owner Clara Wu Tsai is also part of the group.
Rubin and Kraft both lobbied publicly for Mill last year after the Philadelphia-born rapper was sentenced to two-to-four years in prison for probation violations. He was released on bail in April.