The law prohibits state-licensed mental health counselors, including psychologists and social workers, from offering therapy to change sexual orientation in minors.
The Supreme Court in 2014 refused to review the law after an appeals court rejected claims that the ban infringed on free speech rights under U.S. Constitution's the First Amendment.
California outlawed gay conversion therapy in 2012, calling it ineffective and harmful.
New Jersey, Illinois, Oregon, Vermont, New Mexico and the District of Columbia have similar laws on the books, according to the Human Rights Campaign, an advocacy group for lesbian, gay, bisexual and transgender people.
Gay conversion therapy methods range from counseling, hypnosis and dating-skill training to aversive techniques that induce pain or electric shocks in response to same-sex erotic images, according to California officials.
Such treatments come from a belief that homosexuality is a mental illness.
Lead plaintiff Donald Welch, an ordained minister and licensed family therapist, oversees counseling at Skyline Wesleyan Church, an evangelical Christian church in the San Diego area that believes sexuality belongs only in a marriage between a man and a woman.
Welch, along with a Catholic psychiatrist and a man who underwent conversion therapy and now aspires to perform it on others, sued the state claiming the law to ban gay conversion therapy is unconstitutional.