Phillips, 40, was found unconscious just after midnight during a security check by staff at Kern Valley State Prison in Delano outside Bakersfield, the state corrections department said in a statement. He was taken to a hospital where he was pronounced dead more than an hour later.
Phillips had been held in the prison's administrative segregation unit since April.
Last year, Phillips allegedly killed his cellmate at Kern Valley. His trial in that killing was in its early stages in Kern County, officials said.
Phillips was serving a 31-year sentence after he was convicted of domestic violence, false imprisonment and vehicle theft.
Phillips was a standout at the University of
He played with the St. Louis Rams and the
He was arrested at least five times on suspicion of assaulting women, and he was convicted in 2006 of driving a Honda Accord onto a field near the Los Angeles Coliseum and striking three teenagers.