On December 8, 1941, Grace Holmes Carlson, the only female defendant among eighteen Trotskyists convicted under the Smith Act, was sentenced to sixteen months in federal prison for advocating the violent overthrow of the government. After serving a year in Alderson prison, Carlson resumed organizing for the Socialist Workers Party (SWP) and ran for vice president of the United States under its banner in 1948. Then, in 1952, she abruptly left the SWP and returned to the Catholic Church. With the support of the Sisters of St. Joseph, who had educated her as a child, Carlson began a new life as a professor who now advocated for social justice as a Catholic Marxist. The Fierce Life of Grace Holmes Carlson: Catholic, Socialist, Feminist is a historical biography that examines Carlson’s story in the context of her times. Her experiences illuminate the workings of class identity within the context of various influences over the course of a lifespan. Her story contributes to recent historical scholarship exploring the importance of faith in workers’ lives and politics. And it uncovers both the possibilities and limitations for working-class and revolutionary Marxist women in the period between the first- and second-wave feminist movements. The long arc of Carlson’s life (1906–1992) reveals continuities in her political consciousness that transcended the shifts in her partisan commitments, most notably her lifelong dedication to challenging the root causes of social inequality. In that struggle Carlson proved herself to be a truly fierce woman.