In 1942, attorney George Shibley retained McGrath to assist in his defense of 22 Mexican American youths accused in the
Sleepy Lagoon Murder, the largest mass trial in California history. The youths, aged 17 to 21, were accused of killing a Mexican farmworker near a swimming hole in a section of southeast Los Angeles County then known as Sleepy Lagoon. At one event, she raised $1,000 after making a speech to longshoremen in San Francisco. She was also a frequent correspondent with, and visitor of, the Sleepy Lagoon defendants at San Quentin. The committee's supporters included
Orson Welles,
Rita Hayworth,
Nat King Cole, and
Anthony Quinn. While McGrath was working for the Sleepy Lagoon Defense Committee, the group was charged with being a Communist-front organization, and the FBI conducted surveillance of its members. McGrath later recalled, "The FBI would come to neighbors asking questions. I found out much later through the Freedom of Information Act that [the committee's] file was filled with lies. I was much more concerned in those days because they could take anybody and put them away." In October 1944, the Court of Appeal in People v Zamora 66 Cal.App.2d 166, overturned the convictions, finding insufficient evidence of the defendants' guilt, and also pointing to the denial of the defendants' right to counsel and the bias of trial court judge. McGrath was the one who sent the telegram to San Quentin informing the Sleepy Lagoon defendants of the successful appeal. In 1981, McGrath told a
Los Angeles Times interviewer that Sleepy Lagoon appeal was "the most important event in my life. If I had never done anything since ... my involvement in Sleepy Lagoon would justify my existence." ==Portrayals in the media==