His parents Paul and Minna (née Perl) Mosk were Reform Jews (of Hungarian and German origin, respectively) who did not believe in strict religious observances.

[3] Because his father's business in Rockford was floundering, his parents and brother relocated to Los Angeles, and Mosk followed them after graduating from college, as they could not afford to support him in further studies in Chicago.

[citation needed] Upon his inauguration in 1959, Mosk became the first Jew to serve as a statewide executive branch officer in California.

As Attorney General, Mosk issued approximately two thousand written opinions, handled a series of landmark cases, and on January 8, 1962, appeared before the U.S. Supreme Court in Arizona v.

[27] Mosk also commissioned a study of the resurgence of right-wing extremism in California, which famously characterized the secretive John Birch Society as a "cadre" of "wealthy businessmen, retired military officers and little old ladies in tennis shoes.

[citation needed] While an early favorite to be elected to the United States Senate after the death of incumbent Clair Engle, Mosk was appointed to the California Supreme Court in September 1964 by Governor Pat Brown to succeed Roger J. Traynor, who had been elevated to chief justice.

This decision was affirmed by the U.S. Supreme Court in Regents of the University of California v. Bakke, 438 U.S. 265 (1978), which, unlike Mosk's opinion, held that race could be factored in admissions to promote ethnic diversity.

2d 613 (1968):[37] In my years as Attorney General of California (1959–1964), I frequently repeated a personal belief in the social invalidity of the death penalty ...

However, to yield to my predilections would be to act wilfully "in the sense of enforcing individual views instead of speaking humbly as the voice of law by which society presumably consents to be ruled..." (Citation omitted.

[38] As of 2021, Mosk is the last Justice of the California Supreme Court to have served in non-judicial elected office before his appointment to the bench.

On September 27, 1936, he married Helen Edna Mitchell in Beverly Hills, California, and they had one son, Richard.

[40] In 1999, Albany Law School Professor Vincent Martin Bonventre described Mosk as "an institution, an icon, a trailblazer, a legal scholar, a constitutional guardian, a veritable living legend of the American judiciary, ... one of the most influential members in the history of one of the most influential tribunals in the western world.