On March 26, 1951, three years before the historic Brown decision, in Gonzales v. Sheeley (1951), Judge Dave Ling of the United States District Court of Arizona ruled that the segregation of Mexican American students in a separate “Mexican School” was unconstitutional. In this article, we trace the legal arguments in Gonzales through two prior cases, Mendez v. Westminster (1946) and Delgado v. Bastrop (1948). We analyze how racialism, the social science critique of racism and legalism, shaped the arguments in the three cases. Our analysis suggests that Gonzales was a departure from Mendez and Delgado because it was the first case in which a court made an unqualified argument against segregation. The trajectory of the legal arguments across the three cases highlights how new cultural ideas about race were slowly incorporated into civil rights case law, a process that was also shaped by the institutional norms and practices of the legal system.