The death of Phillips, 40, is being investigated as a suicide, the department said.
Phillips was a star running back at Nebraska and a first-round pick for the St. Louis Rams in 1996, but lasted in the NFL only three seasons.
He was sent to a California prison in 2008 after being convicted of domestic violence, false imprisonment and vehicle theft charges.
While serving a 31-year sentence, authorities said, he killed his cellmate in April of last year. A trial for that killing was upcoming in Kern County, California.
His former college coach told the Journal Star newspaper in Lincoln, Nebraska, that he was sad to hear the news about a former player who could have been remembered as an all-time great.
"I saw the potential," Tom Osborne, who coached Nebraska from 1973 to 1997, told the Journal Star
. "I knew this was a guy that could have lived a productive life with the right set of circumstances. For whatever reason it just didn't happen.
"Lawrence obviously had some demons that were never completely put to rest."
Phillips was found unresponsive at 12:05 a.m. Wednesday in his cell at the Kern Valley State Prison, authorities said. He was taken to a hospital, where he was pronounced dead at 1:27 a.m.
The Corrections Department didn't say what led investigators to believe he committed suicide.
The former running back had a huge season as a sophomore for Nebraska, bulldozing his way to 1,772 yards and 16 touchdowns in the 1994 season. The Cornhuskers won the national championship that year and Phillips finished eighth in the Heisman Trophy voting.
But in 1995, he twice got into trouble and was suspended from the team by Osborne. In one of the incidents, Phillips was charged with assaulting his then-girlfriend. He pleaded no contest and received one year of probation.
Later in the season, he was reinstated after going through anger management counseling and was one of the stars of Nebraska's blowout win in the Fiesta Bowl.
The Rams took him with the sixth pick of the 1996 NFL Draft, while he was on probation.
Phillips rushed for 1,453 yards and 14 touchdowns over three seasons
for the Rams, the Miami Dolphins and the San Francisco 49ers in 1996, 1997 and 1999. He added 219 receiving yards and one receiving touchdown.
According to the Los Angeles Times, Phillips also got into legal trouble while with Miami and San Francisco, pleading no contest to misdemeanor battery in Florida and felony assault while with the 49ers.
He also played for the Canadian Football League's Montreal Alouettes and the Calgary Stampeders early last decade, winning the league's championship with the Alouettes in 2002.