Breastfeeding is considered the optimal source of nutrition for infants because of the protection it offers against many adverse maternal and child health consequences. The greatest health benefits are associated with exclusive breastfeeding, but employees must often return to work before establishing a full milk supply. Although federal and California law mandate space and time provisions for workplace lactation accommodation, women employed in manual labor jobs, including agricultural field work, face challenges to expressing (pumping) their breast milk. Mexican-descent women, who represent more than a quarter of the California farmworker population, are particularly at risk for supplementing their breastfed infants with formula. Using a socio-ecological model (SEMBA) to guide the research process, this qualitative case study examined the experiences of five Mexican immigrant women who expressed their milk in the California agricultural workplace. The investigator conducted semi-structured interviews to explore how each level of SEMBA influenced their decision to initiate and then continue pumping in the field. Sociodemographic questionnaires were also completed for each participant. Qualitative content analysis led to the discovery of three themes: Honoring Culture, Running the Gauntlet, and Blazing a Trail. Enabling factors for milk expression in the workplace were found at all levels of SEMBA, except for the work environment. At the individual level, self-efficacy, strengthened by social support (interpersonal), was a major contributor to continuance of milk expression. Significant challenges were found at the interpersonal (coworker) and work environment levels, represented by the theme of Running the Gauntlet. Social support at the interpersonal level was provided by family members, child care providers, and WIC breastfeeding peer counselors and buffered the hardships of the workplace. Community level enablers included cultural values, reflected by the theme of Honoring Culture. Trusting in the (lactation) law at the systems level was mentioned by all study participants as a facilitator for workplace milk expression within the theme of Blazing a Trail. Improved understanding of the factors that enable or challenge success at milk expression on the job may lead to appropriate interventions that support optimal breastfeeding, especially among employees in agriculture or other manual labor jobs.