In late 1918, Warren returned to Oakland, where he accepted a position as the legislative assistant to Leon E. Gray, a newly-elected member of the California State Assembly. Shortly after arriving in the state capital of Sacramento, Warren was appointed as the clerk of the Assembly Judiciary Committee. After a brief stint as a deputy city attorney for Oakland, in 1920 Warren was hired as a deputy district attorney for Alameda County. By the end of 1924, Warren had become the most senior person in the department outside of the district attorney, Ezra Decoto. Though many of his professional colleagues supported Calvin Coolidge, Warren cast his vote for Progressive Party candidate Robert La Follette in the 1924 presidential election. That same year, Warren made his first foray into electoral politics, serving as the campaign manager for his friend, Republican Assemblyman Frank Anderson.