The new law will allow eligible inmates to have their cases reviewed for possible release as early as 15 years into their prison sentence.
Brown signed a bill last year that gave similar rights to juvenile offenders serving life sentences without possibility of parole.
The measure signed by Brown on Monday will cover thousands more inmates who were serving long-term sentences. There are more than 6,500 inmates in state prisons who were under 18 when they committed their crimes but were prosecuted as adults.