Listen to the full interview (7:58)
Listen to an excerpt that aired on CapRadio's Morning Edition (4:06)
Kennedy was in his hometown of Sacramento this week for the dedication of a library and statue in his honor at the federal courthouse.
He says Americans pay too much attention to what he calls the "guilt-or-innocence" process - and don't focus enough on the correctional process within prisons.
"Winston Churchill says a society is judged by how it treats the least deserving of its people," Kennedy says, "and we have to think about that with prisons. Now, there's some very dangerous people in prison that must be confined. But we have to, I think, look very hard at our penal methodologies. I don't think it's working."
Justice Kennedy wrote the 2011 Supreme Court ruling that required California to reduce its prison population. He spoke in an interview Thursday afternoon with Capital Public Radio.