Chelsea's parents, Brent and Kelly King were present at the signing in San Diego, after a seven-month push to change the state law to prevent what happened to Chelsea from happening again.
"We decided the right thing to do for our own healing was to proactively change the laws and to lockup the worst of the worst child predators for life," said Brent King.
The new law has a "one strike" provision requiring a life sentence without parole for forcible sex acts against children. It also tightens sex offense parole guidelines such that some sex offenders will be tracked for life.
Sex offender John Gardner pleaded guilty in April to killing Chelsea and 14-year-old Amber Dubois, who disappeared a year earlier. He received three consecutive terms of life without parole for the two murders and another attack on a jogger last December.
He had been out on parole since 2005, after serving five years for lewd and lascivious acts after attacking a 13-year-old girl.
The Kings said if something like Chelsea's Law had been in effect earlier, their daughter's murder could have been prevented.
"This is something that should have been done a long time ago. If someone can't be rehabilitated then why do we let them out," said Kymberly Koelln, who lives in the Rancho Bernardo community where Chelsea went missing.
But some worry that the longer incarceration and intensive parole oversight it will require could put an untenable financial burden on an already cash-strapped state.
Already this year, the Department of Corrections was forced to cut costs and a ballooning prison population by releasing thousands of non-violent inmates early.
San Diego Assemblyman Nathan Fletcher countered, saying, "We would do all of this and more in a way that added no cost to the state budget by finding reforms within the corrections system."
The Kings, surrounded by community members and remembrances of their daughter, called it a day of celebration.
"You've helped Chelsea fulfill her dreams of changing this world," said Kelly King.